# Human Physiology/The Immune System

## Overview

The immune system is a complex system that is responsible for protecting
us against infections and foreign substances. There are three lines of
defense: the first is to keep invaders out (through skin, mucus
membranes, etc), the second line of defense consists of non-specific
ways to defend against pathogens that have broken through the first line
of defense (such as with inflammatory response and fever). The third
line of defense is mounted against specific pathogens that are causing
disease (B cells produce antibodies against bacteria or viruses in the
extracellular fluid, while T cells kill cells that have become
infected). The immune system is closely tied to the lymphatic system,
with B and T lymphocytes being found primarily within lymph nodes.
Tonsils and the thymus gland are also considered lymph organs and are
involved in immunity. We often don\'t realize how effective the immune
system is until it fails or malfunctions, such as when the lymphocytes
are attacked by HIV in an AIDS patient.

## The Immune System as a Castle

The immune system is a silent wonder. While we are very aware of our
heart beating and the breaths we take, we are much less aware of our
immune system that protects us from thousands of potentially deadly
attacks every day.

In this chapter we will discuss the immune system we each possess that
is working around the clock, protecting us from disease and death.
![](Suecia_3-126_;_Tavastehus_slott.png "Suecia_3-126_;_Tavastehus_slott.png")
A good way to start understanding the immune system is to liken it to a
castle. A castle, like our bodies, is a fortress. A castle has three
lines of defense:

-   *First*, A moat and drawbridge. The first line of defense in our
    bodies are physical and chemical barriers - our skin, stomach acids,
    mucus, tears, vaginal opening, of which the last three mostly
    produce lysozyme to destroy harmful incoming pathogens.
-   *Second*, Sentries and archers who stand on the castle wall. In our
    bodies the second line of defense is non-specific immune responses -
    macrophages, neutrophils, interferons, and complement proteins. This
    line of defense also includes fever and inflammatory response as
    nonspecific defenses.
-   *Third*, Soldiers within the castle. Our third line of defense is
    specific immune responses - T Cells and B Cells. There are many
    types of each which work like a close knit team to destroy
    pathogens.

If pathogens (invaders) try and succeed in penetrating the first line of
defense, then the second line of defense is ready to act. If both the
first and second line of defense fail, then the third line of defense
will act. It is when all three lines of defense are breached that we get
sick and are subject to disease.

So what we are trying to say is that the immune system is a set of
mechanisms of defense, protecting an organism from infection by
identifying and attacking pathogens. This is a difficult task, since
pathogens range from viruses to parasitic worms and must be detected
with absolute specificity as they are \"hidden\" amongst normal cells
and tissues. Pathogens are also constantly changing themselves to avoid
detection and successfully infect and destroy their hosts.

## Lymphatic System

right\|framed\|200px\|The human lymphatic
system

The lymphatic system and the immune system are terms that are used
interchangeably to refer to the body\'s ability to defend against
pathogens. The lymphatic system comprises three interrelated functions:
(1) Removal of excess fluids, lymph, from body tissues, (2) Absorption
of fatty acids and subsequent transport of fat, chyle, to the
circulatory system and (3) Formation of white blood cells (WBCs), and
initiation of immunity through the formation of antibodies, lending
specific resistance to pathogens.

### Lymphatic Pathways

The lymphatic system acts as a secondary circulatory system, except it
collaborates with white blood cells in lymph nodes to protect the body
from being infected by cancer cells, fungi, viruses or bacteria. Unlike
the circulatory system, the lymphatic system is not closed and has no
central pump; the lymph moves slowly and under low pressure due to
peristalsis, the operation of semilunar valves in the lymph veins, and
the milking action of skeletal muscles. Like veins, lymph vessels have
one-way, semi-lunar valves and depend mainly on the movement of skeletal
muscles to squeeze fluid through them. Rhythmic contraction of the
vessel walls may also help draw fluid into the lymphatic capillaries.
This fluid is then transported to progressively larger lymphatic vessels
culminating in the right lymphatic duct (for lymph from the right upper
body) and the thoracic duct (for the rest of the body); these ducts
drain into the circulatory system at the right and left subclavian
veins.

thumb\|left

### Lymph

Lymph originates as blood plasma that leaks from the capillaries of the
circulatory system, becoming interstitial fluid, filling the space
between individual cells of tissue. Plasma is forced out of the
capillaries by hydrostatic pressure, and as it mixes with the
interstitial fluid, the volume of fluid accumulates slowly. Most of the
fluid is returned to the capillaries by osmosis. The proportion of
interstitial fluid that is returned to the circulatory system by osmosis
is about 90% of the former plasma, with about 10% accumulating as
overfill. The excess interstitial fluid is collected by the lymphatic
system by diffusion into lymph capillaries, and is processed by lymph
nodes prior to being returned to the circulatory system. Once within the
lymphatic system the fluid is called lymph, and has almost the same
composition as the original interstitial fluid.

### Edema

Edema is the swelling that forms when too much tissue fluid forms or not
enough taken away. It can be caused by a variety of conditions such as
allergic responses (too much vasodilation), starvation (lack of albumin
in blood lowers osmotic pressure and decreases amount of fluid returning
to capillaries), and lymphatic disorders (e.g. blockage due to parasite
in elephantiasis, or removal of lymph nodes due to a radical
mastectomy). Edema is common in the lower extremities when people spend
a lot of time sitting, because the fluid return is based largely on the
massaging action of skeletal muscles.

### Lymphatic Vessels and Ducts

The lymphatic vessels are similar in structure to the cardiovascular
veins, meaning they also have valves. They are dependent upon the
contraction of skeletal muscle, respiratory movements and valves that do
not allow backward flow. The vessels merge before entering one of two
ducts.

-   Thoracic Duct: This duct is much larger than the lymphatic duct. It
    serves the abdomen, lower extremities and the left side of the upper
    body (head, neck, and arm)
-   Right Lymphatic Duct: This duct serves all of the right side of the
    upper body and thoracic area (head, neck).

## Organs, Tissues and Cells of the Immune System

The immune system consists of a network of lymphatic organs, tissues,
and cells. These structures are supported by the reticuloendothelial
system: loose connective tissue with a network of reticular fibers.
Phagocytic cells, including monocytes and macrophages, are located in
the reticular connective tissue. When micro-organisms invade the body,
or the body encounters antigens (such as pollen), antigens are
transported to the lymph. Lymph is carried through the lymph vessels to
regional lymph nodes. In the lymph nodes, the macrophages and dendritic
cells phagocytose the antigens, process them, and present the antigens
to lymphocytes, which can then start producing antibodies or serve as
memory cells. The function of memory cells is to recognize specific
antigens in the future.

**Primary Lymphatic Organs** The primary lymphatic organs are the red
bone marrow and the thymus. They are the site of production and
maturation of lymphocytes, the type of white blood cell that carries out
the most important work of the immune system.

-   **Red Bone Marrow** Red bone marrow, the soft, spongy, nutrient rich
    tissue in the cavities of certain long bones, is the organ that is
    the site of blood cell production.

Some of the white blood cells produced in the marrow are: neutrophils,
basophils, eosinophils, monocytes, and lymphocytes. Lymphocytes
differentiate into B lymphocytes and T lymphocytes. Red bone marrow is
also the site of maturation of B lymphocytes. T lymphocytes mature in
the thymus. thumb\|Side of thorax, showing surface markings for bones,
lungs (purple), pleura (blue), and spleen
(green).

-   **Thymus Gland** The thymus gland is located in the upper thoracic
    cavity posterior to the sternum and anterior to the ascending aorta.
    The thymus is an organ that is more active in children, and shrinks
    as we get older. Connective tissue separates the thymus into
    lobules, which contain lymphocytes. Thymic hormones such as thymosin
    are produced in the thymus gland. Thymosin is thought to aid in the
    maturation of T lymphocytes. **The Thymus** is critical to the
    immune system. Without a thymus, a person has no ability to reject
    foreign substances, blood lymphocyte level is very poor, and the
    body's response to most antigens is either absent or very weak

Immature T lymphocytes travel from the bone marrow through the
bloodstream to reach the thymus. Here they mature and for the most part,
stay in the thymus. Only 5% of T lymphocytes ever leave the thymus. They
only leave if they are able to pass the test: if they react with "self"
cells, they die. If they have the potential to attack a foreign cell,
they leave the thymus.

**Secondary Lymphatic Organs** The secondary lymphatic organs also play
an important role in the immune system as they are places where
lymphocytes find and bind with antigens. This is followed by the
proliferation and activation of lymphocytes. The secondary organs
include the spleen, lymph nodes, tonsils, Peyer's patches, and the
appendix.

-   **The spleen**, The spleen is a ductless, vertebrate gland that is
    closely associated with the circulatory system, where it functions
    in the destruction of old red blood cells in holding a reservoir of
    blood. Located in the upper left region of the abdominal cavity, it
    is divided into partial compartments. Each compartment contains
    tissue known as white pulp and red pulp. The white pulp contains
    lymphocytes and the red pulp acts in blood filtration. When blood
    enters the spleen and flows through the sinuses for filtration,
    lymphocytes react to pathogens , macrophages engulf debris, and also
    remove old, worn out red blood cells. A person without a spleen is
    more susceptible to infections and may need supplementary antibiotic
    therapy for the rest of their life.

!Structure of the lymph node.**1.** Efferent lymphatic vessel **2.**
Sinus **3.** Nodule **4.** Capsule **5.** Medulla **6.** Valve to
prevent backflow **7.** Afferent lymphatic
vessel.{width="342"}

-   **Lymph Nodes** are small oval shaped structures located along the
    lymphatic vessels. They are about 1-25 mm in diameter. Lymph nodes
    act as filters, with an internal honeycomb of connective tissue
    filled with lymphocytes that collect and destroy bacteria and
    viruses. They are divided into compartments, each packed with B
    lymphocytes and a sinus. As lymph flows through the sinuses, it is
    filtered by macrophages whose function is to engulf pathogens and
    debris. Also present in the sinuses are T lymphocytes, whose
    functions are to fight infections and attack cancer cells. Lymph
    nodes are in each cavity of the body except the dorsal cavity.
    Physicians can often detect the body's reaction to infection by
    feeling for swollen, tender lymph nodes under the arm pits and in
    the neck, because when the body is fighting an infection, these
    lymphocytes multiply rapidly and produce a characteristic swelling
    of the lymph nodes.

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-   **Tonsils** are often the first organs to encounter pathogens and
    antigens that come into the body by mouth or nose. There are 3 pairs
    of tonsils in a ring about the pharynx.

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-   **Peyer's patches**, located in the wall of the intestine and the
    appendix, attached to the cecum of the large intestine, intercept
    pathogens that come into the body through the intestinal tract.

## Leukocytes

The primary cells of the immune system are the *leukocytes* or white
blood cells (WBC). Most leukocytes are much larger than red blood cells,
but they are not nearly as numerous. A microliter of whole blood
contains about 5 million red blood cells but only about 7000 leukocytes.

Although most leukocytes circulate through the blood, they usually leave
the capillaries and function extravascularly (outside the vessels). Some
types of leukocytes can live out in the tissue for several months, but
others may live for only hours or days. Leukocytes can be distinguished
from one another in stained tissue samples by the shape and size of the
nucleus, the staining characteristics of the cytoplasm and the
cytoplasmic inclusions, and the regularity of the cell border.

Leukocytes are divided into six basic types eosinophils, basophils,
neutrophils, monocytes, lymphocytes, and dendritic cells.

One functional group of leukocytes is the **phagocytes**, WBC that
engulf and ingest their targets by phagocytosis. This group includes the
neutrophils, macrophages, monocytes (which are macrophage precursors),
and eosinophils. A second functional group is the **cytotoxic cells**,
so named because they kill the cells they attack. This group includes
eosinophils and some types of lymphocytes.

Lets take a closer look at the six basic types of leukocytes.

**Eosinophils**

Eosinophils fight parasites and contribute to allergic reactions. They
are easily recognized by the bright pink staining granules in their
cytoplasm. Normally, there are only a few eosinophils found in the
peripheral circulatory. They account for only 1-3% of all leukocytes.
The life span of a typical eosinophil in the blood is about 6-12 hours.
Eosinophils are known to attach to large parasites and release
substances from their granules that damage or kill the parasite. Because
eosinophils kill pathogens, they are classified as cytotoxic cells.
Eosinophils also participate in allergic reactions, by contributing to
inflammation and tissue damage by releasing toxic enzymes.

**Basophils**

Basophils release histamine and other chemicals. (Histamine is also
secreted by other cells called MAST cells.) Basophils are rare in
circulation but are easily recognized in a stained blood smear by the
large, dark blue granules in their cytoplasm. They also release
mediators that contribute to inflammation. The granules contain
histamine, heparin(an anticoagulant), cytokines, and other chemicals
involved in allergic and immune responses. !Illistration of a cell
during
phagocytosis.
**Neutrophils**

Neutrophils \"eat\" bacteria and release cytokines. Neutrophils are the
most abundant WBC, 50-70% of the total. They are easily identified by a
segmented nucleus. Neutrophils, like other leukocytes are formed in the
bone marrow. They are phagocytic cells that typically ingest and kill
bacteria. Most neutrophils remain in the blood but can leave the
circulation if attracted to an extravascular site of damage or
infection. In addition to ingesting bacteria and foreign particles,
neutrophils release a variety of cytokines.

**Monocytes**

Monocytes are the precursor cells of tissue macrophages. Monocytes are
not that common in the blood 1-6% of WBC. Once out of the blood,
monocytes enlarge and differentiate into macrophages. Some tissue
macrophages patrol the tissues, creeping along by amoeboid motion.
Others find a location and remain fixed in place. Macrophages are the
primary scavengers within tissues. Macrophages also remove larger
particles, such as old RBC and dead neutrophils. Macrophages play an
important role in the development of acquired immunity. After they
ingest and digest molecular or cellular antigens, fragments of processed
antigen are inserted into the macrophage membrane as part of surface
protein complexes.

**Lymphocytes**

Lymphocytes are the key cells that mediate the acquired immune response
of the body. Only about 5% of lymphocytes are found in circulation. They
constitute 20-30% of all WBC. Most lymphocytes are found in lymphoid
tissues, where they are more likely to encounter invaders. By one
estimate,the adult body contains a trillion lymphocytes at any one time.

**Dendritic Cells**

Dendritic cells activate lymphocytes. They are antigen-presenting cells
characterized by long, thin processes that resemble neuronal dendrites.
Dendritic cells are found in the skin called Langerhans cells and also
in various organs. When dendritic cells recognize and capture antigens,
they migrate to secondary lymphoid tissues, where they present the
antigens to lymphocytes.

## The Three Defenses Against Infection

### Innate Defense -- first line of defense

Physical and chemical barriers are the body\'s first line of defense.

**Physical or Mechanical barriers**

-   **Skin**

One of the body\'s first line of defenses against bacteria and other
harmful organisms is the skin. Our skin is a barrier which stops
infection from entering the body. Millions of microorganisms live
harmlessly on the skin and in the air around us. Sebaceous glands in the
skin produce sweat and sebum, which, combined help to protect the skin.
Both substances contain antiseptic molecules, primarily lysozyme which
breaks down bacterial cell walls. Although our skin is a good defense,
it isn't perfect. The skin itself can also become infected by bacteria,
viruses, fungi or tiny parasites. Some examples of these are: boils,
impetigo; ringworm, athletes foot; cold sore, wart, verucca; and
scabies.

-   **Mucus membranes**

Another very important first line of defense is our mucus membranes. The
mucous membranes (or mucosae; singular: mucosa) line various body
cavities that are exposed to the external environment and internal
organs. It is at several places continuous with skin: at the nostrils,
the lips, the ears, the genital area, and the anus. The nose and mouth
serve as passageways for air going to and from the lungs. As we inhale
and exhale, the mucus membranes that line these passageways warm and
humidify the air. It has been said that there is more bacteria contained
in a human mouth than the sum of all the people that have ever lived on
the earth. Mucus membranes serve different functions, however, their
more important job is to secrete mucus that traps bacteria and other
foreign debris that irritates the lining of the respiratory tract. This
mucus is produced and stored in the sinuses by other mucus membranes. We
get congested when there is excessive fluid in the sinus cavities. This
is a result of an increase in mucus secretions, as well as an increase
in the amount of fluids that passes across the blood vessels of the
mucus membranes that line the nose and sinus. There are also many
chemicals, such as pesticides and anthrax that are absorbed through the
skin. Some mucous membranes are ciliated. *Cilia* are thin, tail-like
projections extending approximately 5--10 micrometers outwards from the
cell body. Their main function is to move things across their surface.

-   **Mucociliary escalator**

The mucociliary clearance of the respiratory tract is an important
defense mechanism against foreign debris and inhaled pathogens. The
cilia that lines the upper and lower airways are lined with a thin layer
of mucus. These beat rapidly to propel particles that are trapped in the
mucus layer to the pharynx. Defective mucociliary clearance predisposes
our respiratory tracts to recurrent infections. These cilial defects may
be either congenital or acquired by infection, toxins or drugs.

**Chemical Defenses**

-   **Tears, saliva**

Tears and saliva contain *lysozyme*, an antiseptic enzyme that attacks
cell walls of bacteria and breaks them down.

-   **Stomach acids**

Glands in the stomach lining produce hydrocloric acid. This acid kills
most invading organisms that are swallowed and take up residence there.

### Non-specific responses to infection - 2nd line of defense

We are born with built in nonspecific defenses that all respond in the
same way to invading pathogens. The outermost defense our body has is
our skin. The sebaceous glands produce sweat and sebum, which contain
antiseptic properties which protect. This bacteria-killing substance
called lyzosome is also found in tears and saliva. Acidic urine in the
urinary tract and friendly bacteria in the genital tract prevent the
multiplying of harmful organisms in these areas. Most invading organisms
in the stomach are killed by gland production of hydrochloric acid.
These are a few examples of how the outer defenses protect us. All outer
defenses work together as the body\'s first line of defense.

#### Inflammatory response

Any break in the skin will allow bacteria to enter the body. These
foreign microbes will cause swelling and reddening at the site of
injury. This reaction by the body is called an *inflammatory reaction*
or *inflammatory response*.

-   Swelling, redness, heat, and pain

Inflammation is characterized by the following quintet: swelling
(tumor), redness (rubor), heat (calor), pain (dolor) and dysfunction of
the organs involved (functio laesa). When an injury occurs, a capillary
and several tissue cells are apt to rupture, releasing histamine and
kinins. These cause the capillaries to dilate, become more permeable,
and leak fluid into these tissues. Dilation and fluid leaking into the
tissues causes swelling, redness, and heat. The swelling and kinins
stimulate nerve endings, causing pain. If there has been a break in the
skin due to the injury, invading microbes may enter. A common cause of
inflammation after surgery is serous fluid. This is a mixture of plasma,
lymph and interstitial fluids seeping from the damaged cells and
vessels. If enough serous fluid accumulates a mass called a seroma may
form. Treatment of a seroma may involve the removal of the fluid with a
needle into a syringe, a process called aspiration.

-   Phagocytosis by neutrophils and macrophages

In the event of a break in the skin, neutrophils, monocytes (and
macrophages) arrive and attempt to engulf and destroy the invaders.
Phagocytosis is receptor-mediated event, which ensures that only
unwanted particles are ingested. Stimulated macrophages can bring about
an explosive increase in the number of leukocytes by producing Colony
Stimulating Factors (CSFs). The CSFs pass by way of the blood to the
bone marrow, where they stimulate the production and the release of
white blood cells (WBCs), primarily neutrophils. Lymphocytes in nearby
lymph nodes produce specific antibodies to attack the microbes. During
the conflict, some neutrophils die and become mixed with dead tissue,
bacteria, living white cells, etc. This thick yellow-white fluid is
called *pus*. When a person has an illness, an examination of the
numbers and types of WBC\'s in their blood can be very useful.

#### Complement System

!Complement protein attacking a cell
membrane.
The complement system is a biochemical cascade of the immune system that
helps clear pathogens from an organism, and promote healing. It is
derived from many small plasma proteins that work together to form the
primary end result of cytolysis by disrupting the target cell\'s plasma
membrane.

Complement is activated by antigen-antibody complexes and causes holes
to form in the plasma membrane of foreign microbes or cells (lysis). The
complement system is considered a nonspecific defense, but it can be
activated against specific microbes that have been marked with
antibodies. Hemolytic transfusion reactions are caused by complement
activation after a person expresses antibodies against the antigens
found on the inappropriately donated blood. Hemolytic Disease of the
Newborn (HDN) is due to maternal antibodies against the Rh factor
crossing the placenta, binding to the baby\'s red blood cells, and
stimulating the baby\'s own complement system to lyse its red blood
cells.

#### Interferon in response to viral infection

Interferon (IFNs) are naturally occurring glycoproteins involved in
non-specific immune responses. Interferons do just as their name states
they \"interfere\" with viral growth. Interferons are initiated from a
cell that has been infected by a virus. When a cell has been infected by
a virus the virus will then cause the cell to make viral nucleic acid.
This nucleic acid acts as a signal and it causes the cell to realize
that it has been infected with a virus. So the cell will start making
and sending out interferons. The IFN\'s that the cell sends out go to
nearby healthy cells and warns them of a virus. The healthy cells then
start intracellular changes that help the cells to be more resistant to
the virus.

### Adaptive Defense (Specific Defense\--third line of defense)

This part of the immune system directly targets invading microbes. Our
specific immune defenses respond to *antigens*. An antigen is a protein
(or polysaccharide) molecule, typically on the cell membrane, that the
body recognizes as *nonself*. They are found on microbes, foreign cells,
or on cancer cells. Normally our immune system does not respond to our
own antigens (if it does, then this is an autoimmune disease). Sometimes
we develop an immune response to a harmless antigen, such as pollen or
cat dander (this is an allergic response).

#### Lymphocytes

Specific immunity is dependent upon two types of lymphocytes, the B
cells and the T cells. Their names are based on where in the body they
mature. B cells mature in the bone marrow, and T cells mature in the
thymus gland. In comparison, both B and T cells can recognize and target
antigen-bearing cells, although they go about this in different ways. B
and T cell lymphocytes are capable of recognizing an antigen because
they have specific receptor molecules on their surface which exactly fit
individual antigens (like a lock and a key). Any B or T cell can only
respond to one type of antigen. The body does not know ahead of time
which antigens it will encounter, but rather makes receptor sites for a
huge number of possible antigens. It is estimated that for the million
or so antigens we encounter in our lifetime we have an equal number of
specific lymphocytes for each possible antigen.

##### B Cells Produce Antibodies

*B cell* lymphocytes are responsible for antibody-mediated immunity
(humoral immunity). They produce antibodies, which are proteins that
bind with and neutralize specific antigens. Antibodies do not directly
kill bacteria, but mark them for destruction. When antibodies bind to
viruses they can prevent the viruses from infecting cells. When
antibodies bind to toxins they can neutralize the toxin (why we get
immunized against the tetanus toxin). Humoral immunity works best
fighting against target viruses, bacteria, and foreign molecules that
are soluble in blood and lymph *before* the bacteria or viruses have
entered into cells (extracellular bacteria and extracellular viruses).

B cells produce two different types of cells:

-   plasma cells
-   memory cells

**Plasma cells**

As B cells mature during embryonic development, they develop surface
receptors that allow them to recognize specific antigens. Then they
travel in the bloodstream, distributing throughout the lymph nodes,
spleen, and tonsils. Once B cells reach their destination, they remain
inactive until they encounter a foreign cell with an antigen that
matches their particular receptor site (most B cells remain inactive for
your entire life). The foreign antigen can be presented to the B cell
directly, but usually macrophages and T cell lymphocytes (helper T
cells) interact with B cells as Antigen Presenting Cells to bring about
antibody production. Upon such an encounter, the B cell\'s receptors
will bind to the antigen. The appropriate B cell is *turned on* or
stimulated. It then grows bigger, and rapidly multiplies into a large
homogenous group (clone). Most of these cells are *plasma cells*, which
actively secrete antibody that will bind with the original stimulating
antigen . While most of the B cells remain in the lymphatic system, the
antibodies are secreted into the lymph fluid which then enters into the
blood plasma to circulate throughout the body. Although the clone cells
only live a few days, their antibodies remain and circulate in the blood
and lymph, gradually decreasing in number.

**Antibody Structure and Function**

There are different classes of antibodies, or immunoglobulins (Ig), such
as IgA, IgG, IgE, and IgM. They can attach to the surface of a microbe
and make it more easily phagocytized by neutrophils, monocytes and
macrophages. Anything that simplifies phagocytosis is called an
*opsonin.* The process of antibodies attaching to invaders can be termed
\'opsonization.\' Some antibodies can bind and inactivate certain
poisons or toxins and are called *antitoxins* (tetanus immunizations
stimulate your body to produce antibodies against the tetanus toxin
rather than against the bacteria that produces the toxin). Still other
antibodies can bind to the surface of microbes and prevent their
attachment to the body\'s cells (thus preventing viruses from entering
host cells). Also, some of them can stimulate nine proteins found in
plasma, called *complement*.

**Memory B cells**

At the time of activation some of the clones become memory B cells.
These cells are long lived and have recorded the information about the
foreign antigen so antibodies can be made more quickly, and in greater
amount, in case a second exposure should occur. Since the second
response is much stronger than the first and puts more antibodies into
circulation, we often receive \"booster shots\" for immunizations.

##### T Cells Attack Infected Cells

thumb\|250px Defending the
body against intracellular pathogens is the role of T lymphocytes, which
carry out *cell-mediated immunity*(CMI). Macrophages phagocytize
invading microbes and present parts of the microbe (antigens) to the T
cell lymphocytes. The appropriate T cell is *turned on* or stimulated.
The activated T cell rapidly multiplies into a large homogenous group
(clone) of *cytotoxic T cells* (Tc cells).

-   \(a\) Attack organisms directly, Also kill infected cells

These cytotoxic T cells migrate to the site of infection (or disease)
and produce chemicals which directly kill the invader. Cytotoxic T cells
release "perforin" that causes pores to form in the plasma membrane of
the target cell, resulting in lysis.

-   \(b\) T cells develop in the thymus gland from immature precursor
    cells that migrate there from the bone marrow.
-   \(c\) Killer and helper T cells
-   \(d\) Memory T Cells

A portion of these activated T cells become *memory* T cells (Tm). These
cells record the information about the foreign antigen so T cells can
respond more quickly, and more strongly, if a second exposure occurs. A
portion of the T cells become *T helper cells* (TH) or *T suppressor
cells* (Ts). TH cell stimulate other T cells and B cells by releasing
*cytokines* and other stimulatory chemicals. Ts cells suppress the
immune response. Experience has shown that cell mediated immunity is
most useful to the body by: Protecting against microbes which exist
inside of our body\'s cells (intracellular bacteria and intracellular
viruses). Protecting against fungal infections. Protecting against
protozoan parasites. Protecting against cancer cells.

## Immune Response Pathways

The innate response starts first, and it is reinforced by the more
specific acquired response. The two pathways are interconnected, so
cooperation and communication is essential.

### Inflammation

What happens when bacteria invade? If the first line of defense fails,
bacteria can reach the extracellular fluid. There they usually cause an
inflammatory response. This response coats antigens on the bacterial
surface, with antibodies. Then in return the antibodies will ingest the
antigens with phagocytic cells. This is characterized by a red, swollen
warm area that is tender or painful. In addition to the nonspecific
inflammatory response, lymphocytes attracted to the area produce
antibodies keyed to the specific type of bacteria. If the infection
continues it will produce a fever.

-   What causes a fever?

During an infection macrophages may release *cytokines* (see glossary),
such as interleukin-1, that travel to the hypothalamus and induce a
change in the *thermostat* setting. When the thermostat is raised to a
new normal temperature, the previous body temperature now registers as
too cold. To increase the temperature to the new level, our body shunts
blood away from the skin (leaving it feeling cold and clammy), the heart
rate increases, and we shiver to generate heat until we reach the new
set point. The hypothalamus may subsequently lower the thermostat, in
which case we suddenly feel hot and start to sweat as our body attempts
to cool off. A person may cycle between chills and sweats during the
course of an infection. While a fever can be dangerous if it gets too
high, or if a patient is weak or has heart trouble, there is some
evidence suggesting that the body may overcome an infection faster if a
fever is allowed to run its course.

### Intracellular Defense

What happens when virus\'s invade the body?

First they encounter an extracellular phase just like the bacteria did.
In the early stages of a viral infection, innate immune responses and
antibodies can help control the invasion of the virus. Once the virus
enters the body\'s host cells cytotoxic T lymphocytes are the main
defense against intracellular viruses. These cells look for infected
host cells, then destroy them.

### Acquired Immunity: Antigen-specific Responses

Acquired immunity responses are antigen-specific responses in which the
body recognizes a foreign substance and selectively reacts to it. This
is mediated primarily by lymphocytes. Acquired immunity overlaps with
the process of innate immunity. Acquired immunity can be subdivided into
active immunity and passive immunity.

**Active Immunity** occurs when the body is exposed to a pathogen and
produces its own antibodies. Active immunity is active because it is the
\"activation\" of your immune system. Active immunity can occur
naturally, when a pathogen invades the body, or artificially, like when
we are given vaccinations containing disabled or killed pathogens. The
body does require prior exposure to an antigen to develop an active
immunity. Some parents expose their children to some antigens so they
will have immunity to these diseases later in life.

**Passive Immunity** occurs when we acquire antibodies made by another
human or animal. Passive immunity is passive because it requires no
response from the person\'s immune system. In passive immunity you are
not presenting the body with foreign antigens. Therefore your immune
system will not need to use B cells, and we know that if the B cells are
never introduced your body isn\'t making antibodies and it isn\'t making
memory B cells. The transfer of antibodies from mother to fetus across
the placenta is one example. Injections containing antibodies are
another. Sometimes travelers going abroad may be injected with gamma
globulin, but this passive immunity last only about three months.
Passive immunizations are used to protect people who have been exposed
to infections or toxins, like snake venom or tetanus.

### Allergic Responses/Inflammatory Responses

An allergy is an inflammatory immune response to a nonpathogenic
antigen. Left alone, the antigen is not harmful to the body, but if
someone is sensitive to the antigen, the body produces an inflammatory
response designed to get rid of it. Allergic inflammatory responses can
range from mild tissue damage to fatal reactions. The immune response in
allergies is called *sensitivity* or *hypersensitivity* to the antigen.
**Immediate hypersensitivity reactions** are mediated (immune
destruction) by antibodies and occur within minutes of exposure to
antigens, which are called allergens. **Delayed hypersensitivity
reactions** are mediated by helper T cells and macrophages and may take
several days to develop.
![](Antigens_presentation.svg "Antigens_presentation.svg") What happens
during a immediate hypersensitivity reaction?

:   1-Foreign protein or antigen is introduced
:   2-Macrophage cell ingests (phagocytosis)
:   3- Activation of Th lymphocyte
:   4-Th (helper) lymphocyte
:   5-Foreign protein bound by membrane antibodies
:   6-B lymphocyte
:   7-Antigen processing (MHC II type)
:   8-Antigen-MHC II complex (antigen presentation)
:   9-Production of antigen-specific antibodies
:   10-Activation of B lymphocyte with active Th

2\. Upon reexposure, the body reacts more strongly and rapidly. The
allergen binds to IgE already present on mast cells, triggering the
immediate release of histamine, cytokines, and other mediators that
cause allergic symptoms. The severity of the reaction varies, ranging
from localized reactions near the site of where the allergen entered,
such as a rash. To the most severe allergic reaction called
*anaphylaxis*. In an anaphylactic reaction, massive release of histamine
and other cytokines cause widespread vasodilation, circulatory collapse,
and severe bronchoconstriction. Unless treated promptly, anaphylaxis can
result in death.

Skin tests for allergies of certain allergens can be injected into the
skin. This is a good way to find out what one might be allergic to so
they can eliminate further exposure. Allergens that can cause immediate
hypersensitivity include bee stings, pollen and certain foods. Allergies
that cause chronic allergic rhinitis and asthma are highly due to dust
mites (dermatophgoides). It is not their bodies that cause the reaction,
but rather it\'s feces. Allergic attacks usually stop when the histamine
has been depleted. This can be stopped faster with an antihistamine drug
or nasal spray.

What happens in a delayed hypersensitivity? It could take hours or days
for symptoms to occur in a delayed hypersensitivity. Delayed
hypersensitivity is cell mediated with a T lymphocyte response.
Secretion of lymphokines, instead of histamine, happens in a delayed
hypersensitivity. So, the treatment would be a corticosteroid instead of
an antihistamine. Examples of a delayed hypersensitivity would be,
poison sumac, poison oak and poison ivy. Skin tests for certain diseases
are also considered examples like TB test and the Mantoux test.

## Infectious Organisms and Immunization

**Beneficial Organisms** **Intestinal bacteria**

-   Bacteria are prokaryotic (before nucleus) cells that we see usually
    as bacilli (rods) or cocci (spheres). While they are the major cause
    of many diseases both fatal and mild, bacteria are also our friends
    and can be of great service to us. Many bacteria in our bodies help
    prevent pathogens from becoming established. \"Good bacteria\" helps
    protect us from \"bad bacteria\". The large intestine is packed with
    *normal microflora* that digest substances otherwise indigestible.
    This process provides our bodies with additional vitamins, fatty
    acids and nutrients. Another example is the microflora that is in
    the vagina that helps maintain an acidic pH, which discourages the
    growth of infectious organisms. These are examples of our immune
    system\'s first line of defense.

**Harmful Organisms**

**Viruses**

-   Viruses are non-living particles consisting of protein and nucleic
    acid that infect cells in biological organisms. They can reproduce
    only by invading and taking over other cells as they lack the
    cellular machinery for self reproduction. A virus is about ten times
    smaller than a bacteria. Some viruses you will recognize are:
    *influenza, herpes, measles, and the common cold*. Some viruses are
    particularly dangerous because they can undergo a period of latency,
    during which they are hidden in the cell and do not reproduce.
    *Influenza* and *HIV* are examples of viruses that frequently
    mutate, thus making it nearly impossible to achieve a long-lasting
    immunity.

**Bacteria**

-   Bacteria can be deadly. They are the major cause of preventable
    infections and death. Some well known illnesses are caused by
    bacteria: *staph infections, strep infections, tuberculosis, food
    poisoning, tetanus, leprosy, and pneumonia*. Because bacterial cells
    are different from human cells, compounds can be found that can kill
    specific bacterial targets while leaving the human patient unharmed.
    Antibacterial agents can be successful in wiping out a bacterial
    infection. The problem with antibiotics is that many strains of
    bacteria are growing resistant to them. Plus, our bodies are not
    getting the chance to develop immunity to certain bacteria. It may
    be better to use probiotics (new supplements that promote the growth
    of healthy and helpful bacteria) rather than depend on antibiotics
    so much.

**Protozoans**

-   The protozoans are mostly eukaryotic unicellular organisms with
    organelles and a nucleus.
-   *Malaria* is the most dangerous disease caused by protozoans and is
    endemic in about 50% of the populations on Earth. Two to four
    million people die each year from malaria, a million of these are
    under the age of five. malaria is caused by one of the protozoan
    genus \"Plasmodium\" which is carried by the female Anopheles
    mosquito.

**Fungi**

-   Fungi are more like animals and humans than they are like bacteria
    because of their eukaryotic cells. Though they produce large,
    visible colonies on old bread, molds and yeasts are in the category
    of microscopic fungi. Yeasts are one-celled and reproduce by
    budding. Molds exist as cell chains, called hyphae.
-   *Mycoses* are diseases caused by fungi. Because of the similarity
    between human cells and fungal cells, it has been difficult for
    scientists to design antibiotics that are effective against fungi
    and do not harm humans. Some of the diseases caused by fungi are:
    *tineas*, *vaginal infection* (candidiasis), and *histoplasmosis*.

**Diagnosis** Infectious diseases are diagnosed by laboratory techniques
such as microscopy and culture. Since many bacteria have no color,
scientists have developed special staining procedures to more accurately
diagnose.

-   Culture

Bacteria and fungi can be identified by growing them on plates until
colonies are visible. Viruses are cultured on eggs or live cells.

-   Antibiotic sensitivity

After colonies of bacteria are grown on plates, discs are placed on the
plates that contain different antibiotics. Bacteria will not grow around
the most effective antibiotic.

-   Tests for viruses

Since viruses are too small to be seen with a light microscope, viral
infections can be diagnosed indirectly by their effects on cells. Some
viruses cause changes to the surface of cultured cells, causing them to
stick together.

**Immunization** While some infectious diseases are common and can occur
many times in the same person, others can only occur once in a lifetime
thanks to the immune system and it\'s ability to remember the organism
and prevent following infections. To avoid an epidemic of a grave
disease such as polio, before the disease can be acquired, an
immunization can create a man-made \"memory\".

-   Active immunization

A person receives an injection (vaccine) that contains dead or harmless
living forms of an organism. The vaccine stimulates the immune system to
produce antibodies and memorize the organism. If there is a later
exposure to this organism and subsequent infection, the antibodies will
stop the infection.

-   Passive immunization

Blood containing antibodies is taken from animals or humans who have
recently had an infection. Blood serum is made that contains the
antibodies, and then injected into the person. The antibodies either
attack an infection that is present or provide short-term protection.

-   Genetically engineered viruses

Genetic engineering is a technique that alters or changes the DNA of a
plant or animal by inserting new genetic information from another
organism. After these organisms replicate, vaccines and hormones are
made that can help fight disease.

-   Hepatitis B vaccine

The gene of the surface antigen of Hepatitis B virus is implanted into
the DNA of a single bacterium. The bacteria produces viral antigens
which are then implanted to stimulate the immune system.

## Immune System Disorders

The immune system is a very complex and highly developed system, yet it
has a very simple mission, seek and destroy invaders. When the immune
system does not function properly it leaves the body open for attacks
from an array of diseases. We classify these into three broad
categories; autoimmunity, immunodeficiencies, and hypersensitivities.

Anything that can trigger the immune response is called an antigen. An
antigen can be a microbe such as a virus, or even a part of a microbe.
Tissues of cells from another person also carry nonself markers and act
as antigens. This explains why tissue transplants can be rejected. In
abnormal situations, the immune system can mistake self for nonself and
launch an attack against the body\'s own cells or tissues. The result is
called an autoimmune disease. Some forms of arthritis and diabetes are
autoimmune diseases. In other cases, the immune system responds to a
seemingly harmless foreign substance such as a dust mite. The result is
allergy, and this kind of antigen is called an allergen.

## The Allergic response

Type 1 hypersensitivity is an allergic reaction provoked by reexposure
to a specific antigen. Exposure may be by ingestion, inhalation,
injection, or direct contact. The reaction is mediated by IgE antibodies
and produced by the immediate release of histamine, tryptase,
arachidonate and derivatives by basophils and mast cells. This causes an
inflammatory response leading to an immediate (within seconds to
minutes) reaction.

The reaction may be either local or systemic. Symptoms vary from mild
irritation to sudden death from anaphylactic shock. Treatment usually
involves epinephrine, antihistamines, and corticosteroids.

-   **Hay Fever**

Hay fever involves an allergic reaction to pollen and results in
allergic rhinitis (inflammation of the nasal mucosa). It is most common
in the haying season, which is why the ailment was named hay fever. A
virtually identical reaction occurs with allergy to mold, animal dander,
dust, and similar inhaled allergens. Particulate matter in polluted air
and chemicals such as chlorine and detergents, which can normally be
tolerated, can greatly aggravate the condition. The pollens that cause
hay fever vary from person to person and from region to region;
generally speaking, the tiny, hardly visible pollens of wind-pollinated
plants are the predominant culprits.

## Autoimmune Disorders

For reasons we do not fully understand, sometimes the immune system
attacks the body the way it normally would attack a germ or foreign
substance. The genes some people inherit can contribute to their
susceptibility to develop an autoimmune disease. Most autoimmune
diseases affect woman more than men.

-   In **Juvenile-onset diabetes** the immune system starts attacking
    and eliminating the cells in the pancreas that make insulin.

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-   **Multiple Sclerosis** is a chronic degenerative disorder of the
    central nervous system where the immune system starts attacking and
    destroying vital myelin in the brain and spinal cord. This causes
    multiple sclerosis (scars) on the myelin sheath resulting in loss of
    nerve function.

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-   Another fairly known disorder is **Rheumatoid Arthritis** this is
    when the immune system starts attacking the tissue inside your
    joints.

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-   There is another disorder, **Organ and Tissue Transplants**, that is
    classified under immuno-deficiencies but in reality is not a failure
    of the immune system. In transplants, foreign tissue is placed
    inside the body. These tissues do not perfectly match the
    surrounding cells. The body sees this as something that should not
    be there and sends messages to attack and kill it. This can make
    transplanting nearly impossible. This problem can not be completely
    prevented but it can be diminished by making sure the donor tissue
    is a close match to the recipient tissue. In addition, the recipient
    is placed on immuno-suppressing drugs to try and prevent the immune
    system from attacking and rejecting the new organ or tissue.

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-   **Vitiligo** is an autoimmune disorder in which the immune system
    destroys pigment-making cells called melanocytes. This results in
    irregularly shaped milky-white patches of skin on different parts of
    the body. This is the condition which Michael Jackson claims to have
    had.

## Immunodeficiency Diseases

When the immune system is presented with foreign antigens in association
with dendritic cells, a vigorous immune response ensues. (Antigens are
the molecules on the surface of invader cells that announce them as
different from the body\'s cells.). Alternatively, dendritic cells can
be exploited during the development of many immune based diseases.

-   **AIDS and HIV**

Acquired immunodeficiency disease (AIDS) is a well-known immune system
disease. Acquired Immune Deficiency Syndrome or Acquired
immunodeficiency syndrome (AIDS or Aids) is a collection of symptoms and
infections resulting from the specific damage to the immune system
caused by the human immunodeficiency virus (HIV).The late stage of the
condition leaves individuals prone to opportunistic infections and
tumors. Although treatments for AIDS and HIV exist to slow the virus\'s
progression, there is no known cure. HIV is transmitted through direct
contact of a mucous membrane or the bloodstream with a bodily fluid
containing HIV, such as blood, semen, vaginal fluid, preseminal fluid,
and breast milk.This transmission can come in the form of anal, vaginal
or oral sex, blood transfusion, contaminated hypodermic needles,
exchange between mother and baby during pregnancy, childbirth, or
breastfeeding, or other exposure to one of the above bodily fluids. AIDS
is the most severe manifestation of infection with HIV. HIV is a
retrovirus that primarily infects vital components of the human immune
system such as CD4+ T cells (a subset of T cells), macrophages and
dendritic cells. It directly and indirectly destroys CD4+ T cells. CD4+
T cells are required for the proper functioning of the immune system.
When HIV kills CD4+ T cells so that there are fewer than 200 CD4+ T
cells per microliter (µL) of blood, cellular immunity is lost, leading
to the condition known as AIDS. Acute HIV infection progresses over time
to clinical latent HIV infection and then to early symptomatic HIV
infection and later to AIDS, which is identified on the basis of the
amount of CD4+ T cells in the blood and the presence of certain
infections.

In the absence of antiretroviral therapy, the median time of progression
from HIV infection to AIDS is nine to ten years, and the median survival
time after developing AIDS is only 9.2 months. However, the rate of
clinical disease progression varies widely between individuals, from two
weeks up to 20 years. Many factors affect the rate of progression. These
include factors that influence the body\'s ability to defend against HIV
such as the infected person\'s general immune function. Older people
have weaker immune systems, and therefore have a greater risk of rapid
disease progression than younger people. Poor access to health care and
the existence of coexisting infections such as tuberculosis also may
predispose people to faster disease progression. The infected person\'s
genetic inheritance plays an important role and some people are
resistant to certain strains of HIV.

## Different Types of T Lymphocyte Cells

Several different subsets of T cells have been described, each with a
distinct function.

***Cytotoxic T cells*** (Tc cells) destroy virally infected cells and
tumor cells, and are also implicated in transplant rejection. These
cells are also known as CD8+ T cells, since they express the CD8
glycoprotein at their surface.

***Helper T cells***, (Th cells) are the \"middlemen\" of the adaptive
immune system. Once activated, they divide rapidly and secrete small
proteins called cytokines that regulate or \"help\" the immune response.
These cells (also called CD4+ T cells) are a target of HIV infection;
the virus infects the cell by using the CD4 protein to gain entry. The
loss of Th cells as a result of HIV infection leads to the symptoms of
AIDS.

***Memory T cells*** are a subset of antigen-specific T cells that
persist long-term after an infection has resolved. They quickly expand
to large numbers of effector T cells upon re-exposure to their cognate
antigen, thus providing the immune system with \"memory\" against past
infections. Memory cells may be either CD4+ or CD8+.

***Regulatory T cells*** (Treg cells), formerly known as suppressor T
cells, are crucial for the maintenance of immunological tolerance. Their
major role is to shut down T cell mediated immunity towards the end of
an immune reaction and to suppress auto-reactive T cells that escaped
the process of negative selection in the thymus. Two major classes of
regulatory T cells have been described, including the naturally
occurring Treg cells and the adaptive Treg cells.

***Treg cells*** (also known as CD4+CD25+FoxP3+ Treg cells) arise in the
thymus, whereas the adaptive Treg cells (also known as Tr1 cells or Th3
cells) may originate during a normal immune response. Naturally
occurring Treg cells can be distinguished from other T cells by the
presence of an intracellular molecule called FoxP3. Mutations of the
FOXP3 gene can prevent regulatory T cell development, causing the fatal
autoimmune disease IPEX.

***\'Natural** ***Killer T cells****\' (NKT cells) are a special kind of
lymphocyte that bridges the adaptive immune system with the innate
immune system. Unlike conventional T cells that recognize peptide
antigen presented by major histocompatibility complex (MHC) molecules,
NKT cells recognize glycolipid antigen presented by a molecule called
CD1d. Once activated, these cells can perform functions ascribed to both
Th and Tc cells (i.e. cytokine production and release of cytolytic/cell
killing molecules).

**THE FUNCTIONS OF T LYMPHOCYTES** T lymphocytes cells help with all
components of the immune system, including cell elimination by killer T
cells and maintaining roles by helper and suppressor T cells. Although
the specific mechanisms of activation vary slightly between different
types of T cells, the \"two-signal model\" in CD4+ T cells holds true
for most.

## The Immune System Pioneers

-   Ilya Mechnikov and the Phagocyte Cells

In 1882, a Russian scientist named Ilya Mechnikov was experimenting with
the larvae of the sea star. He stuck a thorn in the larvae and then he
noticed that something really weird happened. Strange cells started
gathering near the point of insertion. The cells that were surrounding
the thorn were eating any foreign substance that was entering through
the ruptured skin. Mechnikov decided to name these new cells
\"phagocytes\", in Greek meaning \"devouring cells.\"

This discovery was very important since it helped scientists understand
how the body defends itself against disease. If the phagocyte
encountered anything foreign, they attack/arrest or destroy it; all
dependent on the foreign substance. Phagocytes also play an important
part in activating the immune response in the rest of the body.

-   Paul Ehrlich and the Side-chain Theory

Ehrlich supposed that living cells have side-chains. These side chains
can link with a particular toxin, just as Emil Fischer said enzymes must
bind to their receptors \"like a key in a lock.\"

He theorized that a cell under threat grew additional side-chains to
bind the toxin, and that these additional side chains broke off to
become the antibodies that circulate through the body. It was these
antibodies that Ehrlich first described as \"magic bullets\" in search
of toxins.

Ehrlich figured that if a compound could be made that selectively
targeted a disease causing organism, then a toxin for that organism
could be delivered along with the agent of selectivity. Hence, a \"magic
bullet\" would be created that killed only the organism targeted.

Ehrlich predicted autoimmunity calling it \"horror autotoxicus\".

In 1908, Ehrlich and Mechnikov received the Nobel Prize.

## Review Questions

Answers for these questions can be found here

1-When neutrophils and macrophages squeeze out of capillaries to fight
off infection it is called:

:   A\) phagocytosis
:   B\) hemolysis
:   C\) interleukin
:   D\) diapedesis
:   E\) folliculitis

2-During a great battle between your WBC\'s and an aggressive microbe,
an inflammatory response has been initiated. Reddness and edema has
kicked in what else does the body do to protect itself?

:   A\) Histamine cause vasodilation
:   B\) Hypothalmus raises the thermostat
:   C\) Neutrophils engulf and destroy the microbe
:   D\) Living and dead WBC and bacteria accumulate
:   E\) All of the above

3-Specificity and memory are associated with which body defense
mechanism?

:   A\) inflammatory response
:   B\) phagocytosis by macrophages and neutrophils
:   C\) interferon
:   D\) T cell and B cell responses
:   E\) anatomical barriers in the body

4-An additional chemical defense found in tears and saliva?

:   A\) T lymphocytes
:   B\) saline
:   C\) lysozyme
:   D\) EFC

5-Which of the following does complement protein perform

:   A\) They cause antibody release
:   B\) T cell development
:   C\) The release if histamine
:   D\) Promotes tissue repair
:   E\) Mast cell degranulation

6-Which substance induces fever?

:   A\) Pyrogen
:   B\) Pus
:   C\) Monocytes
:   D\) Edema
:   E\) Interferon

7-Major function(s) of the lymphatic system is/are?

:   A\) provide route for return of extracellular fluid
:   B\) act as drain off for inflammatory response
:   C\) render surveillance, recognition , and protection against
    foreign materials via lymphocytes, phagocytes, and antibodies.
:   D\) a and c
:   E\) all of the above

8-An antigen is:

:   A\) a chemical messenger that is released by virus infected cells
:   B\) a lymphocyte responsible for cell-mediated immunity
:   C\) something that coats the inside of lungs, causing infection
:   D\) a protein or other molecule that is recognized as non-self
:   E\) a thick yellow-white fluid

9-A foreign substance, usually a protein, that stimulates the immune
system to react, such as by producing antibodies is a
\_\_\_\_\_\_\_\_\_\_\_\_\_\_.

:   A\) allergen
:   B\) antigen
:   C\) histamine
:   D\) mast cell
:   E\) interferon

10-When a macrophage ingests an invading bacteria and takes the antigen
to a lymph node, what happens next?

:   A\) the macrophage will present it to the first B-cell it
    encounters, and the B-cell will in turn change its surface receptors
    to match the antigen
:   B\) a B-cell will only become activated if it already has a match
    for the antigen
:   C\) a matching B-cell will become activated into a cytotoxic T-cell
:   D\) the cells of the lymph node will release histamine
:   E\) the lymph node will increase production of neutrophils

11-What is the most common portal of entry for diseases, into the body?

:   A\) Respiratory system
:   B\) Endocrine system
:   C\) Hematacrit system
:   D\) Any opening into the body.

12-This gland shrinks in size during adulthood, and has hormones that
function in maturation of T-lymphocytes:

:   A\) lymph nodes
:   B\) thymus
:   C\) spleen
:   D\) GALT
:   E\) tonsils

13-Which of the following is not a mechanical factor to protect the skin
and mucous membranes from infection?

:   A\) Layers of cells
:   B\) Tears
:   C\) Saliva
:   D\) Lysozyme
:   E\) None of the above

14-Where is the site of maturation for a B cell?

:   A\) thymus
:   B\) bone marrow
:   C\) pancreas
:   D\) cortex

15-Nonspecific resistance is

:   A\) The body\'s ability to ward off diseases.
:   B\) The body\'s defenses against any kind of pathogen.
:   C\) The body\'s defense against a particular pathogen.
:   D\) The lack of resistance.
:   E\) None of the above.

16\. What is an Antibody?

:   A\) An antimicrobial substance applied to a living tissue to prevent
    infection.
:   B\) Programmed cell death
:   C\) A protein generated by the immune system in response to a
    foreign substance.
:   D\) A chemical involved in inflammation.

# Glossary

**Antibody**: Antibody or (immunoglobulin) is a protein generated by the
immune system (B cells) in response to a foreign substance (antigen).

**Antibody titer**: A test done to check the immunity of vaccination,
when identification of a low immunity to a vaccine a booster shot can be
given to increase the immunity.

**Antigen**: Protein (or polysaccharide) molecule that the body
recognizes as nonself. Substance body recognizes as foreign such as,
fungi, viruses, protozoans, parasitic worms, pollen, poison ivy plant
resin, insect venom, and transplanted organs.

**Antiseptic**: Antimicrobial substance applied to living tissue or skin
to prevent infection.

**Apoptosis**: Programmed cell death

**B Cell**: Lymphocytes that are responsible for antibody-mediated
immunity

**Basophils**: WBC that release histamine and other chemicals

**Chemotaxis**: Movement of cells, phagocytes especially, they move in a
specific direction in a tumbling fashion like rolling this is all due to
a chemical stimulant.

**Complement System**: Biochemical cascade of the immune system that
helps clear pathogens from an organism, and promote healing

**Cytokines**: Regulatory peptides that control cell
development,differentiation, and the immune response

**Dendritic**: cells that activate lymphocytes

**Diapedesis**: The movement of WBC\'s from the blood to the surrounding
tissue. A mechanism of the kind phagocyte that will walk or crawl out of
the blood stream to site of infection.

**Edema**: Swelling that forms when too much tissue fluid forms or not
enough taken away

**Eosinophils**: WBC that fight parasites and contribute to allergic
reactions

**Histamine**: Histamine is a chemical involved in inflammation, this
chemical makes capillaries leaky, in this it will move more fluid out
into the tissue spaces.

**IgA**: Found in breast milk, mucus, saliva, and tears. This
immunoglobulin functions to stop the pathogens before entry to the
internal environment.

**IgD**: This immunoglobulin is found on B-cells and function is not
known.

**IgE**: This immunoglobulin is combined with mast cells that in turn
release histamine, this kind of globulin is released in the presence of
an allergic response or parasitic infection.

**IgG**: This immunoglobulin is the majority of the specific immunity
against bacteria and viruses in the extracellular fluid.

**IgM**: This immunoglobulin is associated to antibodies that react to
incompatibility of ABO and Rh factor grouping.

**Immunoglobulins**: Proteins that are antibodies receptors on the
surface of B-cells, there are five classes.

**Kinins**: Kinins is a chemical involved in inflammation, it is
inactive in blood plasma but become activated by tissue damage and in
turn stimulate pain receptors in skin.

**Leukocytes**: primary cells of the immune system; also called white
blood cells

**Lymph**: fluid of the lymph system; originates as blood plasma that
leaks from the capillaries of the circulatory system, becoming
interstitial fluid, filling the space between individual cells of tissue

**Lymphocytes**: The key cells that mediate the acquired immune response
of the body

**Lymph Nodes**: Small oval shaped structures located along the
lymphatic vessels

**Lysosome**: Organelle containing digestive enzymes (acid hydrolases)
that digest viruses, bacteria, food particles and worn out organelles

**Lysozyme**: Enzyme that attacks cell walls of bacteria and breaks them
down; found and used as an antiseptic property in the body\'s first line
of defense (ie. saliva, tears, sweat, etc)

**Macrophages**: WBC that are the primary scavengers within tissues

**Membrane Attack Complex (MAC)**: Work in the same way as the perforins
of the NK cells that is it punches holes in the membrane that causes
lysis.

**Neutrophils**: WBC that \"eat\" bacteria and release cytokines

**Opsonin**: Any substance that promotes a phagocytosis by binding a
microbe to a phagocyte.

**Perforin**: Protein secreted by cytotoxic T cells, causes pores to
form in the plasma membrane of the target cell resulting in lysis.

**Peyer's Patches**: located in the wall of the intestine and the
appendix, attached to the cecum of the large intestine, intercept
pathogens that come into the body through the intestinal tract

**Phagocytes**: WBC that engulf and ingest their targets by phagocytosis

**Pyrogens**: Foreign substances and or microorganisms that causes
hypothalamic thermoregulatory center to increase and causes fever
(pyrexia)

**Right Lymphatic Duct**: Lymphatic duct that serves all of the right
side of the upper body and thorasic area(head,neck)

**Spleen**: Ductless, vertebrate gland that is closely associated with
the circulatory system, where it functions in the destruction of old red
blood cells in holding a reservoir of blood

**T Cell**: cells that carry out cell-mediated immunity

**Thoracic Duct**: Lymphatic duct that serves the abdomen, lower
extremities and the left side of the upper body(head,neck, and arm)

**Thymus Gland**: Gland that contains lymphocytes; produces thymosin
that is thought to aid in the maturation of T lymphocytes