Understanding the


Immune System
 

How It Works
 

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES 

NATIONAL INSTITUTES OF HEALTH 

National Institute of Allergy and Infectious Diseases 
Artwork by Jeanne Kelly 
Understanding the

Immune System 

How It Works 

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES 

NATIONAL INSTITUTES OF HEALTH 

National Institute of Allergy and Infectious Diseases 

NIH Publication No. 07-5423 

September 2007 

www.niaid.nih.gov 
Contents
 

1 Introduction 

2 Self and Nonself 

3 The Structure of the Immune System 

7 Immune Cells and Their Products 

19 Mounting an Immune Response 

25 Immunity: Natural and Acquired 

28 Disorders of the Immune System 

34 Immunology and Transplants 

35 The Immune System and 

the Nervous System 

36 Frontiers in Immunology 

42 Summary 

43 Glossary 

Note: Words in bold are defined in the glossary at the end 

of this booklet. 
Introduction
 

T

he immune system is a network of cells, 

tissues, and organs that work together to 

defend the body against attacks by “foreign” 

invaders. These are primarily microbes—tiny 

organisms such as bacteria, parasites, and 

fungi that can cause infections. Viruses also 

cause infections, but are too primitive to be 

classified as living organisms. The human 

body provides an ideal environment for many 

microbes. It is the immune system’s job to 

keep them out or, failing that, to seek out and 

destroy them. 

When the immune system hits the wrong 

target, however, it can unleash a torrent of 

disorders, including allergic diseases, arthritis, 

and a form of diabetes. If the immune system 

is crippled, other kinds of diseases result. 

The immune system is amazingly complex. 

It can recognize and remember millions of 

different enemies, and it can produce secretions 

(release of fluids) and cells to match up with 

and wipe out nearly all of them. 

The secret to its success is an elaborate and 

dynamic communications network. Millions 

and millions of cells, organized into sets and 

subsets, gather like clouds of bees swarming 

around a hive and pass information back 

and forth in response to an infection. Once 

immune cells receive the alarm, they become 

activated and begin to produce powerful 

chemicals. These substances allow the cells 

to regulate their own growth and behavior, 

enlist other immune cells, and direct the new 

recruits to trouble spots. 

Bacteria: 

streptococci 

Virus: 

herpes virus 

Parasite: 

schistosome 

Fungus: 

penicillium mold 

1 
Self and Nonself 

T

he key to a healthy immune system is its 

remarkable ability to distinguish between 

the body’s own cells, recognized as “self,” 

and foreign cells, or “nonself.” The body’s 

immune defenses normally coexist peacefully 

with cells that carry distinctive “self” marker 

molecules. But when immune defenders 

encounter foreign cells or organisms carrying 

markers that say “nonself,” they quickly 

launch an attack. 

Anything that can trigger this immune 

response is called an antigen. An antigen can 

be a microbe such as a virus, or a part of a 

microbe such as a molecule. Tissues or cells 

from another person (except an identical twin) 

also carry nonself markers and act as foreign 

antigens. This explains why tissue transplants 

may be rejected. 

Antigen Marker molecule Antibody

Antigens carry marker molecules that identify them 

as foreign. 

2 
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 ragweed 

pollen. The result is allergy, and this kind of 

antigen is called an allergen. 

The Structure of the Immune System 

T

he organs of the immune system are 

positioned throughout the body. They are 

called lymphoid organs because they are 

home to lymphocytes, small white blood 

cells that are the key players in the immune 

system. Bone marrow, the soft tissue in the 

hollow center of bones, is the ultimate source 

of all blood cells, including lymphocytes. 

The thymus is a lymphoid organ that lies 

behind the breastbone. Lymphocytes known 

as T lymphocytes or T cells (“T” stands for 

“thymus”) mature in the thymus and then 

migrate to other tissues. B lymphocytes, 

also known as B cells, become activated 

and mature into plasma cells, which make 

and release antibodies. 

3 
Tonsils and 

adenoids 

Lymph

nodes 

Lymph 

nodes 

Lymphatic 

vessels 

Appendix

Thymus 

Spleen 

Peyer’s 

patches 

Bone 

marrow 

Lymph 

nodes 

Lymphatic 

vessels 

The organs of the immune system are positioned throughout the body. 

4 
Germinal 

center 

Follicle 

Medulla 

Vein 

Artery 

Outgoing 

lymph vessel 

Incoming 

lymph vessel 

Paracortex 

Cortex 

The lymph node contains numerous specialized structures. 

T cells concentrate in the paracortex, B cells in and around 

the germinal centers, and plasma cells in the medulla. 

Lymph nodes, which are located in many 

parts of the body, are lymphoid tissues that 

contain numerous specialized structures. 

•	 T cells from the thymus concentrate in 

the paracortex. 

•	 B cells develop in and around the 

germinal centers. 

•	 Plasma cells occur in the medulla. 

Lymphocytes can travel throughout the body 

using the blood vessels. The cells can also travel 

through a system of lymphatic vessels that 

closely parallels the body’s veins and arteries. 

5 
Lymph 

node 

Lymphatic 

vessel 

Immune cells and foreign particles enter the lymph nodes 

via incoming lymphatic vessels or the lymph nodes’ tiny 

blood vessels. 

Cells and fluids are exchanged between blood 

and lymphatic vessels, enabling the lymphatic 

system to monitor the body for invading 

microbes. The lymphatic vessels carry lymph, 

a clear fluid that bathes the body’s tissues. 

Small, bean-shaped lymph nodes are laced 

along the lymphatic vessels, with clusters in 

the neck, armpits, abdomen, and groin. Each 

lymph node contains specialized compartments 

where immune cells congregate, and where 

they can encounter antigens. 

Immune cells, microbes, and foreign antigens 

enter the lymph nodes via incoming lymphatic 

vessels or the lymph nodes’ tiny blood vessels. 

All lymphocytes exit lymph nodes through 

outgoing lymphatic vessels. Once in the 

bloodstream, lymphocytes are transported to 

tissues throughout the body. They patrol 

6 
everywhere for foreign antigens, then gradually 

drift back into the lymphatic system to begin 

the cycle all over again. 

The spleen is a flattened organ at the upper 

left of the abdomen. Like the lymph nodes, 

the spleen contains specialized compartments 

where immune cells gather and work. The 

spleen serves as a meeting ground where 

immune defenses confront antigens. 

Other clumps of lymphoid tissue are found 

in many parts of the body, especially in the 

linings of the digestive tract, airways, and 

lungs—territories that serve as gateways to 

the body. These tissues include the tonsils, 

adenoids, and appendix. 

Immune Cells and Their Products 

T

he immune system stockpiles a huge 

arsenal of cells, not only lymphocytes but 

also cell-devouring phagocytes and their 

relatives. Some immune cells take on all 

intruders, whereas others are trained on 

highly specific targets. To work effectively, 

most immune cells need the cooperation 

of their comrades. Sometimes immune cells 

communicate by direct physical contact, 

and sometimes they communicate releasing 

chemical messengers. 

The immune system stores just a few of each 

kind of the different cells needed to recognize 

millions of possible enemies. When an antigen 

first appears, the few immune cells that can 

7 
respond to it multiply into a full-scale army 

of cells. After their job is done, the immune 

cells fade away, leaving sentries behind to 

watch for future attacks. 

All immune cells begin as immature stem 

cells in the bone marrow. They respond 

to different cytokines and other chemical 

signals to grow into specific immune cell 

types, such as T cells, B cells, or phagocytes. 

Because stem cells have not yet committed 

to a particular future, their use presents 

an interesting possibility for treating some 

immune system disorders. Researchers 

currently are investigating if a person’s own 

stem cells can be used to regenerate damaged 

immune responses in autoimmune diseases 

and in immune deficiency disorders, such as 

HIV infection. 

Antigenbinding 

site 

Heavy 

chain 

Light 

chain 

Variable region 

Constant region 

An antibody is made 

up of two heavy 

chains and two light 

chains. The variable 

region, which differs 

from one antibody to 

the next, allows an 

antibody to recognize 

its matching antigen. 

B Cells 

B cells and T cells are the main types of 

lymphocytes. B cells work chiefly by secreting 

substances called antibodies into the body’s 

fluids. Antibodies ambush foreign antigens 

circulating in the bloodstream. They are 

powerless, however, to penetrate cells. The 

job of attacking target cells—either cells that 

have been infected by viruses or cells that 

have been distorted by cancer—is left to T 

cells or other immune cells (described below). 

Each B cell is programmed to make one specific 

antibody. For example, one B cell will make 

an antibody that blocks a virus that causes 

the common cold, while another produces an 

antibody that attacks a bacterium that causes 

8 
B cell 

Plasma cell Antibody 

B cells mature into plasma cells that produce antibodies. 

pneumonia. When a B cell encounters the kind 

of antigen that triggers it to become active, it 

gives rise to many large cells known as plasma 

cells, which produce antibodies. 

•	 Immunoglobulin G, or IgG, is a kind of 

antibody that works efficiently to coat 

microbes, speeding their uptake by other 

cells in the immune system. 

•	 IgM is very effective at killing bacteria. 

•	 IgA concentrates in body fluids—tears, 

saliva, and the secretions of the respiratory 

and digestive tracts—guarding the entrances 

to the body. 

•	 IgE, whose natural job probably is to 

protect against parasitic infections, is 

responsible for the symptoms of allergy. 

9 
•	 IgD remains attached to B cells and 

plays a key role in initiating early B cell 

responses. 

Immunoglobulins 

T Cells 

Unlike B cells, T cells do not recognize 

free-floating antigens. Rather, their surfaces 

contain specialized antibody-like receptors 

that see fragments of antigens on the surfaces 

of infected or cancerous cells. T cells 

contribute to immune defenses in two major 

ways: some direct and regulate immune 

responses, whereas others directly attack 

infected or cancerous cells. 

Helper T cells, or Th cells, coordinate 

immune responses by communicating with 

other cells. Some stimulate nearby B cells 

to produce antibodies, others call in microbegobbling cells called phagocytes, and still 

others activate other T cells. 

Cytotoxic T lymphocytes (CTLs)—also called 

killer T cells—perform a different function. 

These cells directly attack other cells carrying 

certain foreign or abnormal molecules on 

their surfaces. CTLs are especially useful for 

attacking viruses because viruses often hide 

from other parts of the immune system while 

they grow inside infected cells. CTLs recognize 

small fragments of these viruses peeking out 

from the cell membrane and launch an attack 

to kill the infected cell. 

In most cases, T cells only recognize an 

antigen if it is carried on the surface of a 

cell by one of the body’s own major 

histocompatibility complex, or MHC, 

10 
molecules. MHC molecules are proteins 

recognized by T cells when they distinguish 

between self and nonself. A self-MHC 

molecule provides a recognizable scaffolding 

to present a foreign antigen to the T cell. 

In humans, MHC antigens are called human 

leukocyte antigens, or HLA. 

Although MHC molecules are required for 

T cell responses against foreign invaders, 

they also create problems during organ 

transplantations. Virtually every cell in the 

body is covered with MHC proteins, but each 

person has a different set of these proteins 

on his or her cells. If a T cell recognizes a 

nonself-MHC molecule on another cell, it 

will destroy the cell. Therefore, doctors must 

match organ recipients with donors who 

have the closest MHC makeup. Otherwise 

the recipient’s T cells will likely attack the 

transplanted organ, leading to graft rejection. 

Mature helper 

T cell 

Mature cytotoxic 

T cell 

Immature T cell 

Some T cells are helper cells; others are killer cells. 

11 
Killer cell Target cell 

Surface contact 

Target-oriented 

granules 

Killer cell makes contact with target cell, trains its weapons 

on the target, then strikes. 

Natural killer (NK) cells are another kind of 

lethal white cell, or lymphocyte. Like CTLs, 

NK cells are armed with granules filled with 

potent chemicals. But CTLs look for antigen 

fragments bound to self-MHC molecules, 

whereas NK cells recognize cells lacking selfMHC molecules. Thus, NK cells have the 

potential to attack many types of foreign cells. 

Both kinds of killer cells slay on contact. The 

deadly assassins bind to their targets, aim 

their weapons, and then deliver a lethal burst 

of chemicals. 

T cells aid the normal processes of the 

immune system. If NK T cells fail to function 

properly, asthma, certain autoimmune 

diseases—including type 1 diabetes—or the 

12 
growth of cancers may result. NK T cells 

get their name because they are a kind of T 

lymphocyte that carries some of the surface 

proteins, called “markers,” typical of NK T 

cells. But these T cells differ from other kinds 

of T cells. They do not recognize pieces of 

antigen bound to self-MHC molecules. Instead, 

they recognize fatty substances (lipids and 

glycolipids) that are bound to a different class 

of molecules called CD1d. Scientists are trying 

to discover methods to control the timing and 

release of chemical factors by NK T cells, with 

the hope they can modify immune responses 

in ways that benefit patients. 

Phagocytes and Their Relatives 

Phagocytes are large white cells that can 

swallow and digest microbes and other 

foreign particles. Monocytes are phagocytes 

that circulate in the blood. When monocytes 

migrate into tissues, they develop into 

macrophages. Specialized types of macrophages 

can be found in many organs, including the 

lungs, kidneys, brain, and liver. 

Macrophages play many roles. As scavengers, 

they rid the body of worn-out cells and other 

debris. They display bits of foreign antigen in 

a way that draws the attention of matching 

lymphocytes and, in that respect, resemble 

dendritic cells (see page 15). And they churn 

out an amazing variety of powerful chemical 

signals, known as monokines, which are 

vital to the immune response. 

Granulocytes are another kind of immune 

cell. They contain granules filled with potent 

chemicals, which allow the granulocytes to 

13 
Monocyte 

Macrophage 

Mast cell 

Basophil 

Neutrophil 

Eosinophil 

Phagocytes, granulocytes, and mast cells, all with 

different methods of attack, demonstrate the immune 

system’s versatility. 

destroy microorganisms. Some of these 

chemicals, such as histamine, also contribute 

to inflammation and allergy. 

One type of granulocyte, the neutrophil, is 

also a phagocyte. Neutrophils use their 

prepackaged chemicals to break down the 

microbes they ingest. Eosinophils and 

basophils are granulocytes that “degranulate” 

by spraying their chemicals onto harmful cells 

or microbes nearby. 

Mast cells function much like basophils, 

except they are not blood cells. Rather, they 

are found in the lungs, skin, tongue, and 

linings of the nose and intestinal tract, where 

they contribute to the symptoms of allergy. 

14 
Related structures, called blood platelets, are 

cell fragments. Platelets also contain granules. In 

addition to promoting blood clotting and wound 

repair, platelets activate some immune defenses. 

Dendritic cells are found in the parts of 

lymphoid organs where T cells also exist. 

Like macrophages, dendritic cells in lymphoid 

tissues display antigens to T cells and help 

stimulate T cells during an immune response. 

They are called dendritic cells because they 

have branchlike extensions that can interlace 

to form a network. 

T Cell Receptors 

T cell receptors are complex protein molecules 

that peek through the surface membranes 

of T cells. The exterior part of a T cell receptor 

recognizes short pieces of foreign antigens that 

are bound to self-MHC molecules on other 

cells of the body. It is because of their T cell 

receptors that T cells can recognize diseasecausing microorganisms and rally other 

immune cells to attack the invaders, or kill 

the invaders themselves. 

Toll-like receptors (TLRs), which occur on 

cells throughout the immune system, are a 

family of proteins the body uses as a first line 

of defense against invading microbes. Like T 

cell receptors, some TLRs peek through the 

surface membranes of immune cells, allowing 

them to respond to microbes in the cells’ 

environment. 

Some TLRs are activated by molecules that 

make up viruses, whereas other TLRs respond 

to molecules that make up the cell walls of 

15 
bacteria. Once activated, TLRs relay the alarm 

to other actors in the immune system. For 

example, some TLRs play important roles in 

the all-purpose “first-responder” arm of the 

immune system, also called the innate immune 

system. In short order, the innate immune 

system responds with a surge of chemical 

signals that together cause inflammation, fever, 

and other responses to infection or injury. 

Other TLRs help initiate responses from 

genetically identical groups of lymphocytes, 

called clones, that are already programmed to 

recognize specific antigens. Such responses 

are called adaptive immunity. 

Overall, the cellular receptors important for 

the first-line responses of innate immunity are 

encoded by genes people inherit from their 

parents. In contrast, adaptive immune responses 

rely on antigen receptors that are pieced 

together in the genomes of lymphocytes during 

their development in various tissues of the 

body. In addition to TLRs, other kinds of innate 

immune receptors can stimulate phagocytosis 

by macrophages, trigger the inflammatory 

responses that help control local infections, 

and play a range of crucial roles in defending 

the body against invading microbes. 

Cytokines 

Cells of the immune system communicate 

with one another by releasing and responding 

to chemical messengers called cytokines. 

These proteins are secreted by immune cells 

and act on other cells to coordinate appropriate 

immune responses. Cytokines include a diverse 

assortment of interleukins, interferons, and 

growth factors. 

16 
Mature helper T cell Macrophage 

Lymphokines Monokines

Cytokines include lymphokines, produced by lymphocytes, 

and monokines, made by monocytes and macrophages. 

Some cytokines are chemical switches that 

turn certain immune cell types on and off. 

One cytokine, interleukin 2 (IL-2), triggers the 

immune system to produce T cells. IL-2’s 

immunity-boosting properties have traditionally 

made it a promising treatment for several 

illnesses. Clinical studies are underway to 

test its benefits in diseases such as cancer, 

hepatitis C, and HIV infection and AIDS. 

Scientists are studying other cytokines to 

see whether they can also be used to treat 

diseases. 

One group of cytokines chemically attracts 

specific cell types. These so-called chemokines 

are released by cells at a site of injury or 

infection and call other immune cells to the 

region to help repair the damage or fight off 

the invader. Chemokines often play a key 

role in inflammation and are a promising 

target for new drugs to help regulate immune 

responses. 

17 
Complement 

The complement system is made up of 

about 25 proteins that work together to assist, 

or “complement,” the action of antibodies in 

destroying bacteria. Complement also helps 

to rid the body of antibody-coated antigens 

(antigen-antibody complexes). Complement 

Antigen 

Antibody (IgG) 

First complement protein 

Complement cascade 

Cylindrical complex 

inserted in cell wall 

Cell 

swells 

and 

bursts 

The interlocking steps of the complement cascade end 

in cell death. 

18 
proteins, which cause blood vessels to become 

dilated and then leaky, contribute to the redness, 

warmth, swelling, pain, and loss of function 

that characterize an inflammatory response. 

Complement proteins circulate in the blood in 

an inactive form. When the first protein in the 

complement series is activated—typically by 

antibody that has locked onto an antigen—it 

sets in motion a domino effect. Each component 

takes its turn in a precise chain of steps known 

as the complement cascade. The end products 

are molecular cylinders that are inserted into— 

and that puncture holes in—the cell walls that 

surround the invading bacteria. With fluids and 

molecules flowing in and out, the bacterial 

cells swell, burst, and die. Other components 

of the complement system make bacteria 

more susceptible to phagocytosis or beckon 

other immune cells to the area. 

Mounting an Immune Response 

I

nfections are the most common cause of 

human disease. They range from the common 

cold to debilitating conditions like chronic 

hepatitis to life-threatening diseases such as 

AIDS. Disease-causing microbes (pathogens) 

attempting to get into the body must first move 

past the body’s external armor, usually the skin 

or cells lining the body’s internal passageways. 

The skin provides an imposing barrier to 

invading microbes. It is generally penetrable 

only through cuts or tiny abrasions. The 

digestive and respiratory tracts—both portals 

of entry for a number of microbes—also have 

19 
Complement 

Macrophage 

Virus 

Lymphokines 

T cell 

Antibodies 

B cell 

Killer cell 

When challenged by a virus or other microbe, the immune 

system has many weapons to choose. 

their own levels of protection. Microbes 

entering the nose often cause the nasal surfaces 

to secrete more protective mucus, and attempts 

to enter the nose or lungs can trigger a sneeze 

or cough reflex to force microbial invaders out 

of the respiratory passageways. The stomach 

contains a strong acid that destroys many 

pathogens that are swallowed with food. 

If microbes survive the body’s front-line 

defenses, they still have to find a way through 

the walls of the digestive, respiratory, or 

urogenital passageways to the underlying cells. 

These passageways are lined with tightly 

packed epithelial cells covered in a layer of 

mucus, effectively blocking the transport of 

many pathogens into deeper cell layers. 

20 
Mucosal surfaces also secrete a special class 

of antibody called IgA, which in many cases 

is the first type of antibody to encounter an 

invading microbe. Underneath the epithelial 

layer a variety of immune cells, including 

macrophages, B cells, and T cells, lie in wait 

for any microbe that might bypass the barriers 

at the surface. 

Next, invaders must escape a series of general 

defenses of the innate immune system, which 

are ready to attack without regard for specific 

antigen markers. These include patrolling 

phagocytes, NK T cells, and complement. 

Microbes cross the general barriers then confront 

specific weapons of the adaptive immune 

system tailored just for them. These specific 

weapons, which include both antibodies and 

T cells, are equipped with singular receptor 

structures that allow them to recognize and 

interact with their designated targets. 

Bacteria, Viruses, and Parasites 

The most common disease-causing microbes 

are bacteria, viruses, and parasites. Each uses a 

different tactic to infect a person, and, therefore, 

each is thwarted by different components of the 

immune system. 

Most bacteria live in the spaces between cells 

and are readily attacked by antibodies. When 

antibodies attach to a bacterium, they send 

signals to complement proteins and phagocytic 

cells to destroy the bound microbes. Some 

bacteria are eaten directly by phagocytes, which 

signal to certain T cells to join the attack. 

21 
The combination of antigen 

fragment and MHC 

molecule attracts

the help of a 

mature, matching

T cell. 

Antibodies are triggered when a B cell 

encounters its matching antigen. 

The B cell takes in the antigen 

and digests it, 

Lymphokines secreted by 

the T cell allow the B cell to 

multiply and mature into 

antibody-producing plasma cells.

then it displays antigen fragments 

bound to its own distinctive 

MHC molecules. 

Released into the bloodstream, antibodies lock onto 

matching antigens. These antigen-antibody complexes 

are soon eliminated, either by the complement cascade 

or by the liver and the spleen. 

B cells are triggered to mature into plasma cells that 

produce a specific kind of antibody when the B cell 

encounters a specific antigen. 

22 
T cells are mobilized 

when they encounter a cell such as a macrophage 

or a B cell that has digested an antigen 

and is displaying antigen fragments 

bound to its MHC molecules. 

Lymphokines 

help the T cell 

to mature. 

The T cell, alerted 

and activated, 

secretes 

lymphokines. 

Some lymphokines 

attract immune cells— 

fresh macrophages, 

granulocytes, and 

other lymphocytes— 

to the site of infection. 

Yet other lymphokines 

direct the recruits once 

they arrive on the scene. Some T cells 

become killer cells 

and track down 

body cells infected 

by viruses. 

Infected 

cells 

Some 

lymphokines 

spur the 

growth of 

more T cells. 

T cells become active through a series of steps and then 

activate other immune cells by secreting lymphokines. 

23 
All viruses, plus a few types of bacteria and 

parasites, must enter cells of the body to 

survive, requiring a different kind of immune 

defense. Infected cells use their MHC molecules 

to put pieces of the invading microbes on 

their surfaces, flagging down CTLs to destroy 

the infected cells. Antibodies also can assist 

in the immune response by attaching to and 

clearing viruses before they have a chance to 

enter cells. 

Parasites live either inside or outside cells. 

Intracellular parasites such as the organism 

that causes malaria can trigger T cell responses. 

Extracellular parasites are often much larger 

than bacteria or viruses and require a much 

broader immune attack. Parasitic infections 

often trigger an inflammatory response in 

which eosinophils, basophils, and other 

specialized granule-containing cells rush to 

the scene and release their stores of toxic 

chemicals in an attempt to destroy the 

invaders. Antibodies also play a role in this 

attack, attracting the granule-filled cells to 

the site of infection. 

24 
�������

Natural Natural 

Acquired Acquired

�����

Immunity: Natural and Acquired 

L

ong ago, physicians realized that people who 

had recovered from the plague would never 

get it again—they had acquired immunity. This 

is because some of the activated T and B cells 

had become memory cells. Memory cells 

ensure that the next time a person meets up 

with the same antigen, the immune system 

is already set to demolish it. 

Immunity can be strong or weak, short-lived 

or long-lasting, depending on the type of 

antigen it encounters, the amount of antigen, 

and the route by which the antigen enters 

the body. Immunity can also be influenced 

by inherited genes. When faced with the 

same antigen, some individuals will respond 

forcefully, others feebly, and some not at all. 

25 
An immune response can be sparked not 

only by infection but also by immunization 

with vaccines. Some vaccines contain 

microorganisms—or parts of microorganisms— 

that have been treated so they can provoke an 

immune response but not full-blown disease. 

(See “Vaccines” on page 27.) 

Immunity can also be transferred from one 

individual to another by injections of serum 

rich in antibodies against a particular microbe 

(antiserum). For example, antiserum is 

sometimes given to protect travelers to 

countries where hepatitis A is widespread. 

The antiserum induces passive immunity 

against the hepatitis A virus. Passive immunity 

typically lasts only a few weeks or months. 

Infants are born with weak immune responses 

but are protected for the first few months of 

life by antibodies they receive from their 

mothers before birth. Babies who are nursed 

can also receive some antibodies from breast 

milk that help to protect their digestive tracts. 

Immune Tolerance 

Immune tolerance is the tendency of T or B 

lymphocytes to ignore the body’s own tissues. 

Maintaining tolerance is important because it 

prevents the immune system from attacking 

its fellow cells. Scientists are hard at work 

trying to understand how the immune system 

knows when to respond and when to ignore 

an antigen. 

26 
Tolerance occurs in at least two ways— 

central tolerance and peripheral tolerance. 

Central tolerance occurs during lymphocyte 

development. Very early in each immune 

cell’s life, it is exposed to many of the self 

molecules in the body. If it encounters these 

molecules before it has fully matured, the 

encounter activates an internal self-destruct 

pathway, and the immune cell dies. This 

process, called clonal deletion, helps ensure 

that “self-reactive” T cells and B cells, those 

that could develop the ability to destroy the 

body’s own cells, do not mature and attack 

healthy tissues. 

Because maturing lymphocytes do not 

encounter every molecule in the body, they 

must also learn to ignore mature cells and 

tissues. In peripheral tolerance, circulating 

lymphocytes might recognize a self molecule 

but cannot respond because some of the 

chemical signals required to activate the T 

or B cell are absent. So-called clonal anergy, 

therefore, keeps potentially harmful 

lymphocytes switched off. Peripheral 

tolerance may also be imposed by a special 

class of regulatory T cells that inhibits helper 

or cytotoxic T-cell activation by self antigens. 

Vaccines 

For many years, healthcare providers have 

used vaccination to help the body’s immune 

system prepare for future attacks. Vaccines 

consist of killed or modified microbes, 

components of microbes, or microbial DNA 

that trick the body into thinking an infection 

has occurred. 

27 
A vaccinated person’s immune system 

attacks the harmless vaccine and prepares 

for invasions against the kind of microbe the 

vaccine contained. In this way, the person 

becomes immunized against the microbe. 

Vaccination remains one of the best ways 

to prevent infectious diseases, and vaccines 

have an excellent safety record. Previously 

devastating diseases such as smallpox, polio, 

and whooping cough have been greatly 

controlled or eliminated through worldwide 

vaccination programs. 

Disorders of the Immune System 

Allergic Diseases 

The most common types of allergic diseases 

occur when the immune system responds to 

a false alarm. In an allergic person, a normally 

harmless material such as grass pollen, food 

particles, mold, or house dust mites is mistaken 

for a threat and attacked. 

Allergies such as pollen allergy are related 

to the antibody known as IgE. Like other 

antibodies, each IgE antibody is specific; one 

acts against oak pollen and another against 

ragweed, for example. 

Autoimmune Diseases 

Sometimes the immune system’s recognition 

apparatus breaks down, and the body begins 

to manufacture T cells and antibodies directed 

against self antigens in its own cells and 

tissues. As a result, healthy cells and tissues 

28 
Ragweed The first time the allergy-prone 

pollen person runs across an allergen 

such as ragweed, B cell 

IgE 

he or she makes large amounts 

of ragweed IgE antibody. 

Plasma 

cell 

These IgE molecules attach 

themselves to mast cells. 

Mast


cell
 

The second time that person 

has a brush with ragweed, 

the IgE-primed mast cell will 

release its powerful chemicals, 

Chemicals 

Symptoms 

and the person will suffer the 

wheezing and/or sneezing, 

runny nose, watery eyes, and 

itching of allergy. 

An allergic reaction occurs after plasma cells produce 

IgE antibody against a specific antigen and mast cells 

become activated. 

29 
Mature T cell 

Beta cell 

Pancreas 

Misguided T cells can attack insulin-producing cells 

of the pancreas, contributing to an autoimmune form 

of diabetes. 

are destroyed, which leaves the person’s 

body unable to perform important functions. 

Misguided T cells and autoantibodies, as they 

are known, contribute to many autoimmune 

diseases. For instance, T cells that attack certain 

kinds of cells in the pancreas contribute to a 

form of diabetes, whereas an autoantibody 

known as rheumatoid factor is common in 

people with rheumatoid arthritis. People with 

systemic lupus erythematosus (SLE) have 

antibodies to many types of their own cells and 

cell components. SLE patients can develop a 

severe rash, serious kidney inflammation, and 

disorders of other important tissues and organs. 

No one knows exactly what causes an 

autoimmune disease, but multiple factors are 

likely to be involved. These include elements 

in the environment, such as viruses, certain 

drugs, and sunlight, all of which may damage 

30 
or alter normal body cells. Hormones are 

suspected of playing a role because most 

autoimmune diseases are far more common in 

women than in men. Heredity, too, seems to 

be important. Many people with autoimmune 

diseases have characteristic types of selfmarker molecules. 

Immune Complex Diseases 

Immune complexes are clusters of interlocking 

antigens and antibodies. Normally, immune 

complexes are rapidly removed from the 

bloodstream. Sometimes, however, they 

continue to circulate and eventually become 

trapped in the tissues of the kidneys, lungs, 

skin, joints, or blood vessels. There, they set 

off reactions with complement that lead to 

Small complex 

Endothelial cell 

Glomerular basement 

membrane of kidney 

Large 

complex 

Antigen-antibody complexes can become trapped in, 

and damage, the kidneys and other organs. 

31 
inflammation and tissue damage. Immune 

complexes work their mischief in many diseases. 

These include malaria and viral hepatitis, as 

well as many autoimmune diseases. 

Immune Deficiency Disorders 

When the immune system is missing one 

or more of its parts, the result is an immune 

deficiency disorder. These disorders can be 

inherited, acquired through infection, or 

produced as a side effect by drugs such as 

those used to treat people with cancer or 

those who have received transplants. 

Temporary immune deficiencies can develop 

in the wake of common virus infections, 

including influenza, infectious mononucleosis, 

and measles. Immune responses can also be 

depressed by blood transfusions, surgery, 

malnutrition, smoking, and stress. 

Some children are born with poorly functioning 

immune systems. Some have flaws in the B 

cell system and cannot produce antibodies. 

Others, whose thymus is either missing or 

small and abnormal, lack T cells. Very rarely, 

infants are born lacking all of the major 

immune defenses. This condition is known as 

severe combined immune deficiency disease 

or SCID. (See “Gene Therapy” on page 39.) 

32 
Virus components Virus DNA 

Cell DNA 

New 

virus 

New AIDS 

virus budding 

from T cell 

T cell 

The AIDS virus takes over the machinery of the T cells it 

infects, using the cells to make new viruses. 

AIDS is an immune deficiency disorder 

caused by a virus (HIV) that infects immune 

cells. HIV can destroy or disable vital T cells, 

paving the way for a variety of immunologic 

shortcomings. The virus also can hide out for 

long periods in immune cells. As the immune 

defenses falter, a person develops AIDS and 

falls prey to unusual, often life-threatening 

infections and rare cancers. 

33 
Immunology and Transplants 


E

ach year thousands of lives in the United 

States are prolonged by transplanted organs 

including the kidneys, heart, lung, liver, and 

pancreas. For a transplant to “take,” however, 

the body’s natural tendency to rid itself of 

foreign tissue must be overridden. 

One way to avoid the rejection of transplanted 

tissue is tissue typing, which ensures that 

markers of self on the donor’s tissue are as 

similar as possible to those of the recipient. 

Every cell in the body has a double set of 

six major tissue antigens, and each of the 

antigens exists, in different individuals, in 

as many as 20 varieties. The chance of two 

people having identical transplant antigens 

is about one in 100,000. 

A second way to avoid transplant rejection 

is to lull the recipient’s immune system into 

a less active state. This can be done with 

powerful immunosuppressive drugs such 

as cyclosporine A, or by using laboratorymanufactured antibodies that attack mature 

T cells. 

Bone Marrow Transplants 

When the immune response is severely 

depressed—in infants born with immune 

disorders or in people with cancer, for 

example—one possible remedy is a transfer 

of healthy bone marrow. Once introduced 

into the circulation, transplanted bone 

marrow cells can develop into functioning 

B and T cells. 

34 
In bone marrow transplants, a close match 

is extremely important. Not only is there a 

danger that the body will reject the transplanted 

bone marrow cells, but mature T cells from 

the bone marrow transplant may counterattack 

and destroy the recipient’s tissues. To prevent 

this situation, known as graft-versus-host 

disease, scientists use drugs or antibodies to 

“cleanse” the donor marrow of potentially 

dangerous mature T cells. 

The Immune System and the Nervous System 

E

vidence is mounting that the immune 

system and the nervous system are linked 

in several ways. One well-known connection 

involves the adrenal glands. In response to 

stress messages from the brain, the adrenal 

glands release hormones into the blood. In 

addition to helping a person respond to 

emergencies by mobilizing the body’s energy 

reserves, these “stress hormones” can stifle 

the protective effects of antibodies and 

lymphocytes. 

Another link between the immune system 

and the nervous system is that the hormones 

and other chemicals that convey messages 

among nerve cells also “speak” to cells of 

the immune system. Indeed, some immune 

cells are able to manufacture typical nerve 

cell products, and some lymphokines can 

transmit information to the nervous system. 

Moreover, the brain may send messages 

directly down nerve cells to the immune 

system. Networks of nerve fibers have been 

found connecting to the lymphoid organs. 

35 
Frontiers in Immunology 

Scientists are now able to mass-produce 

immune cell secretions, both antibodies and 

lymphokines, as well as specialized immune 

cells. The ready supply of these materials 

not only has revolutionized the study of the 

immune system itself but also has had an 

enormous impact on medicine, agriculture, 

and industry. 

Monoclonal antibodies are identical antibodies 

made by the many clones of a single B cell. 

Antibody-producing 

plasma cell 

Monoclonal antibodies 

Long-lived 

plasma cell 

Cells fuse into 

a hybridoma 

Antigen 

Monoclonal antibody technology makes it possible to 

mass produce specific antibodies to order. 36 
Because of their unique specificity for 

different antigens, monoclonal antibodies are 

promising treatments for a range of diseases. 

Researchers make monoclonal antibodies by 

injecting a mouse with a target antigen and 

then fusing B cells from the mouse with other 

long-lived cells. The resulting hybrid cell 

becomes a type of antibody factory, turning 

out identical copies of antibody molecules 

specific for the target antigen. 

Mouse antibodies are “foreign” to people, 

however, and might trigger an immune response 

when injected into a human. Therefore, 

researchers have developed “humanized” 

monoclonal antibodies. To construct these 

molecules, scientists take the antigen-binding 

portion of a mouse antibody and attach it to a 

human antibody scaffolding, greatly reducing 

the foreign portion of the molecule. 

Because they recognize very specific molecules, 

monoclonal antibodies are used in diagnostic 

tests to identify invading pathogens or changes 

in the body’s proteins. In medicine, monoclonal 

antibodies can attach to cancer cells, blocking 

the chemical growth signals that cause the 

cells to divide out of control. In other cases, 

monoclonal antibodies can carry potent 

toxins into certain cells, killing the dangerous 

cells while leaving their neighbors untouched. 

Genetic Engineering 

Genetic engineering allows scientists to pluck 

genes—segments of DNA—from one type of 

organism and combine them with genes of a 

second organism. In this way, relatively simple 

37 
Cytokine-producing 

cell 

Bacterium 

Strand of DNA 

from cytokineproducing cell 

Plasmid—a ring 

of DNA—from 

bacterium 

Cytokine gene 

is cut out of 

DNA 

Plasmid is cut 

open 

Cytokine gene 

is spliced into 

plasmid 

Hybrid plasmid 

is put back into 

bacterium 

Bacterium makes 

human cytokines 

Genetic engineering transforms simple organisms into 

factories for making human proteins. 

38 
organisms such as bacteria or yeast (a type 

of fungus) can be induced to make quantities 

of human proteins, including hormones such as 

insulin as well as lymphokines and monokines. 

They can also manufacture proteins from 

infectious agents, such as the hepatitis virus 

or HIV, for use in vaccines. 

Gene Therapy 

Genetic engineering also holds promise for gene 

therapy—replacing altered or missing genes or 

adding helpful genes. One disease in which 

gene therapy has been successful is SCID, or 

severe combined immune deficiency disease. 

SCID is a rare genetic disease that disables 

a person’s immune system and leaves the 

person unable to fight off infections. It is 

caused by mutations in one of several genes 

that code for important components of the 

immune system. Until recently, the most 

effective treatment for SCID was transplantation 

of blood-forming stem cells from the bone 

marrow of a healthy person who is closely 

related to the patient. However, doctors have 

also been able to treat SCID by giving the 

patient a genetically engineered version of 

the missing gene. 

Using gene therapy to treat SCID is generally 

accomplished by taking blood-forming cells 

from a person’s own bone marrow, introducing 

into the cells a genetically changed virus that 

carries the corrective gene, and growing the 

modified cells outside the person’s body. After 

the genetically changed bone marrow cells 

39 
begin to produce the enzyme or other protein 

that was missing, the modified blood-forming 

marrow cells can be injected back into the 

person. Once back inside the body, the 

genetically modified cells can produce the 

missing immune system component and 

begin to restore the person’s ability to fight 

off infections. 

Cancer is another target for gene therapy. 

In pioneering experiments, scientists are 

removing cancer-fighting lymphocytes from 

the cancer patient’s tumor, inserting a gene 

that boosts the lymphocytes’ ability to make 

quantities of a natural anticancer product, 

then growing the restructured cells in quantity 

in the laboratory. These cells are injected 

back into the person, where they can seek 

out the tumor and deliver large doses of 

the anticancer chemical. 

Immunoregulation 

Research into the delicate checks and balances 

that control the immune response is increasing 

knowledge of normal and abnormal immune 

system functions. Someday it may be possible 

to treat autoimmune diseases such as systemic 

lupus erythematosus by suppressing parts of 

the immune system that are overactive. 

40 
Immatur

human 

immune 

tissue

e 

Immature 

human 

immune 

cells 

Mouse kidneys 

Immuno-incompetent SCID-hu mouse 

The SCID-hu mouse provides a means of studying the 

human immune system in action. 

Scientists are also devising ways to better 

understand the human immune system and 

diseases that affect it. For example, by 

transplanting immature human immune 

tissues or immune cells into SCID mice, 

scientists have created “humanized” mice, a 

living model of the human immune system. 

Scientists are manipulating the immune 

system of humanized SCID mice to discover 

ways to benefit human health. Humanized 

mice are also being used in research on 

transplantation and autoimmune and allergic 

diseases, and to manufacture molecules that 

help regulate immune system function and 

immune tolerance. 

41 
Summary 

Although scientists have learned much 

about the immune system, they continue 

to study how the body launches attacks that 

destroy invading microbes, infected cells, 

and tumors while ignoring healthy tissues. 

New technologies for identifying individual 

immune cells are now allowing scientists to 

determine quickly which targets are triggering 

an immune response. Improvements in 

microscopy are permitting the first-ever 

observations of living B cells, T cells, and 

other cells as they interact within lymph 

nodes and other body tissues. 

In addition, scientists are rapidly unraveling 

the genetic blueprints that direct the human 

immune response, as well as those that 

dictate the biology of bacteria, viruses, and 

parasites. The combination of new technology 

and expanded genetic information will no 

doubt reveal even more about how the body 

protects itself from disease. 

42 
Glossary 

adenoids—see tonsils. 

adrenal gland—a gland located on each 

kidney that secretes hormones regulating 

metabolism, sexual function, water balance, 

and stress. 

allergen—any substance that causes an 

allergy. 

antibody—a molecule (also called an 

immunoglobulin) produced by a mature B cell 

(plasma cell) in response to an antigen. When 

an antibody attaches to an antigen, it helps the 

body destroy or inactivate the antigen. 

antigen—a substance or molecule that is 

recognized by the immune system. The antigen 

can be from foreign material such as bacteria 

or viruses. 

antiserum—a serum rich in antibodies against 

a particular microbe. 

appendix—lymphoid organ in the intestine. 

artery—a blood vessel that carries blood from 

the heart to other parts of the body. 

autoantibody—an antibody that reacts against 

a person’s own tissue. 

autoimmune disease—disease that results 

when the immune system mistakenly attacks 

the body’s own tissues. Examples include 

multiple sclerosis, type 1 diabetes, rheumatoid 

arthritis, and systemic lupus erythematosus. 

43 
B cell or B lymphocyte—a small white blood 

cell crucial to the immune defenses. B cells 

come from bone marrow and develop into 

blood cells called plasma cells, which are the 

source of antibodies. 

bacteria—microscopic organisms composed 

of a single cell. Some cause disease. 

basophil—a white blood cell that contributes 

to inflammatory reactions. Along with mast 

cells, basophils are responsible for the 

symptoms of allergy. 

blood vessel—an artery, vein, or capillary 

that carries blood to and from the heart and 

body tissues. 

cell—the smallest unit of life; the basic living 

unit that makes up tissues. 

chemokine—a small protein molecule that 

activates immune cells, stimulates their 

migration, and helps direct immune cell 

traffic throughout the body. 

clonal anergy—the process of switching off 

the ability of potentially harmful T or B cells 

to participate in immune responses. Clonal 

anergy is essential for generating the tolerance 

of T and B cells to the body’s “self” tissue 

antigens. 

44 
clonal deletion—the genetically controlled 

process of eliminating immune cells that 

could destroy the body’s own cells and 

tissues. The elimination process removes 

immature T and B lymphocytes that have 

receptors for cells with “self” MHC or HLA 

antigens, and could therefore attack and 

destroy the body’s own cells. 

clone—a group of genetically identical cells 

or organisms descended from a single common 

ancestor; or, to reproduce identical copies. 

complement—a complex series of blood 

proteins whose action “complements” the 

work of antibodies. Complement destroys 

bacteria, produces inflammation, and 

regulates immune reactions. 

complement cascade—a precise sequence of 

events, usually triggered by antigen-antibody 

complexes, in which each component of the 

complement system is activated in turn. 

cytokines—powerful chemical substances 

secreted by cells that enable the body’s cells 

to communicate with one another. Cytokines 

include lymphokines produced by lymphocytes 

and monokines produced by monocytes and 

macrophages. 

cytotoxic T lymphocyte (CTL)—a subtype of 

T cells that carries the CD8 marker and can 

destroy body cells infected by viruses or 

transformed by cancer. 

45 
dendritic cell—an immune cell with highly 

branched extensions that occurs in lymphoid 

tissues, engulfs microbes, and stimulates 

T cells by displaying the foreign antigens 

of the microbes on their surfaces. 

DNA (deoxyribonucleic acid)—a long molecule 

found in the cell nucleus. Molecules of DNA 

carry the cell’s genetic information. 

enzyme—a protein produced by living cells 

that promotes the chemical processes of life 

without itself being altered. 

eosinophil—a white blood cell containing 

granules filled with chemicals damaging 

to parasites and enzymes that affect 

inflammatory reactions. 

epithelial cells—cells that make up the 

epithelium, the covering for internal and 

external body surfaces. 

fungus—a member of a class of relatively 

primitive vegetable organisms. Fungi include 

mushrooms, yeasts, rusts, molds, and smuts. 

gene—a unit of genetic material (DNA) 

inherited from a parent that controls specific 

characteristics. Genes carry coded directions 

a cell uses to make specific proteins that 

perform specific functions. 

genome—a full set of genes in a person or 

any other living thing. 

46 
graft rejection—an immune response against 

transplanted tissue. 

graft-versus-host disease—a life-threatening 

reaction in which transplanted cells attack the 

tissues of the recipient. 

granule—a membrane-bound organelle 

(specialized part) within cells where proteins 

are stored before secretion. 

granulocyte—a phagocytic white blood cell 

filled with granules. Neutrophils, eosinophils, 

basophils, and mast cells are examples of 

granulocytes. 

growth factors—chemicals secreted by cells 

that stimulate proliferation of or changes in 

the physical properties of other cells. 

helper T cells (Th cells)—a subset of T cells 

that carry the CD4 surface marker and are 

essential for turning on antibody production, 

activating cytotoxic T cells, and initiating 

many other immune functions. 

hepatitis—the name of several viruses that 

cause liver diseases. These viruses include 

hepatitis A, hepatitis B, and hepatitis C. 

histocompatibility testing—a test conducted 

before transplant operations to find a donor 

whose MHC molecules are similar to the 

recipient’s; helps reduce the strength of 

transplant rejection. 

47 
HIV (human immunodeficiency virus)—the 

virus that causes AIDS. 

human leukocyte antigen (HLA)—a protein 

on the surfaces of human cells that identifies 

the cells as “self” and, like MHC antigens, 

performs essential roles in immune responses. 

HLAs are used in laboratory tests to determine 

whether one person’s tissues are compatible 

with another person’s, and could be used in 

a transplant. HLAs are the human equivalent 

of MHC antigens; they are coded for by 

MHC genes. 

immune response—reaction of the immune 

system to foreign substances. Although 

normal immune responses are designed to 

protect the body from pathogens, immune 

dysregulation can damage normal cells and 

tissues, as in the case of autoimmune diseases. 

immunoglobulin—one of a family of large 

protein molecules, also known as antibodies, 

produced by mature B cells (plasma cells). 

immunosuppressive—capable of reducing 

immune responses. 

inflammation—an immune system reaction 

to “foreign” invaders such as microbes or 

allergens. Signs include redness, swelling, 

pain, or heat. 

inflammatory response—redness, warmth, and 

swelling produced in response to infection; the 

result of increased blood flow and an influx of 

immune cells and their secretions. 

48 
innate—an immune system function that is 

inborn and provides an all-purpose defense 

against invasion by microbes. 

interferon—a protein produced by cells that 

stimulates antivirus immune responses or 

alters the physical properties of immune cells. 

interleukins—a major group of lymphokines 

and monokines. 

lymph—a transparent, slightly yellow fluid 

that carries lymphocytes, bathes the body 

tissues, and drains into the lymphatic vessels. 

lymph node—a small bean-shaped organ of 

the immune system, distributed widely 

throughout the body and linked by lymphatic 

vessels. Lymph nodes are garrisons of B and 

T cells, dendritic cells, macrophages, and 

other kinds of immune cells. 

lymphatic vessels—a bodywide network of 

channels, similar to the blood vessels, which 

transports lymph to the immune organs and 

into the bloodstream. 

lymphocyte—a small white blood cell produced 

in the lymphoid organs and essential to 

immune defenses. B cells, T cells, and NK T 

cells are lymphocytes. 

49 
lymphoid organ—an organ of the immune 

system where lymphocytes develop and 

congregate. These organs include the bone 

marrow, thymus, lymph nodes, spleen, and 

various other clusters of lymphoid tissue. 

Blood vessels and lymphatic vessels are 

also lymphoid organs. 

lymphokines—powerful chemical 

substances secreted by lymphocytes. These 

molecules help direct and regulate the 

immune responses. 

macrophage—a large and versatile immune 

cell that devours invading pathogens and 

other intruders. Macrophages stimulate 

other immune cells by presenting them 

with small pieces of the invaders. 

major histocompatibility complex (MHC)— 

a group of genes that controls several 

aspects of the immune response. MHC 

genes code for “self” markers on all 

body cells. 

mast cell—a granulocyte found in tissue. 

The contents of mast cells, along with 

those of basophils, are responsible for the 

symptoms of allergy. 

memory cells—a subset of T cells and 

B cells that have been exposed to antigens 

and can then respond more readily when 

the immune system encounters those 

same antigens again. 

50 
microbe or microorganism—a microscopic 

living organism. Examples include bacteria, 

protozoa, and some fungi and parasites. 

Viruses are also called microbes. 

molecule—the smallest amount of a specific 

chemical substance. Large molecules such as 

proteins, fats, carbohydrates, and nucleic acids 

are the building blocks of a cell, and a gene 

determines how each molecule is produced. 

monoclonal antibody—an antibody produced 

by a single B cell or its identical progeny that 

is specific for a given antigen. Monoclonal 

antibodies are used as research tools for binding 

to specific protein molecules, and are invaluable 

in research, medicine, and industry. 

monocyte—a large phagocytic white blood 

cell which, when entering tissue, develops 

into a macrophage. 

monokines—powerful chemical substances 

secreted by monocytes and macrophages. 

These molecules help direct and regulate the 

immune responses. 

natural killer (NK) cell—a large granulecontaining lymphocyte that recognizes and 

kills cells lacking self antigens. These cells’ 

target recognition molecules are different 

from T cells. 

NK T cell—a T cell that has some 

characteristics of NK cells. It produces large 

amounts of cytokines when stimulated, and 

is activated by fatty substances (lipids) bound 

to non-MHC molecules called CD1d. 

51 
neutrophil—a white blood cell that is an 

abundant and important phagocyte. 

organ—a part of the body that has a specific 

function, such as the lungs. 

organism—an individual living thing composed 

of one or more cells. 

parasite—a plant or animal that lives, grows, 

and feeds on or within another living organism. 

passive immunity—immunity resulting from 

the transfer of antibodies or antiserum 

produced by another person. 

pathogen—a disease-causing organism or virus. 

phagocyte—a large white blood cell that 

contributes to immune defenses by ingesting 

microbes or other cells and foreign particles. 

phagocytosis—process by which one cell 

engulfs another cell or large particle. 

plasma cell—a large antibody-producing cell 

that develops from B cells. 

platelet—a cellular fragment critical for blood 

clotting and sealing off wounds. 

serum—the clear liquid that separates from 

the blood when it is allowed to clot. This 

fluid contains the antibodies that were present 

in the whole blood. 

52 
spleen—a lymphoid organ in the abdominal 

cavity that is an important center for immune 

system activities. 

stem cell—an immature cell from which other 

cells derive. Bone marrow is rich in the kind of 

stem cells that become specialized blood cells. 

T cell or T lymphocyte—a small white blood 

cell that recognizes antigen fragments bound 

to cell surfaces by specialized antibody-like 

receptors. “T” stands for the thymus gland, 

where T cells develop and acquire their 

receptors. 

T cell receptor—complex protein molecule on 

the surfaces of T cells that recognizes bits of 

foreign antigen bound to self-MHC molecules. 

Toll-like receptor (TLR)—a family of proteins 

important for first-line immune defenses 

against microbes. 

tissue typing—see histocompatibility testing. 

tissue—a group of similar cells joined to 

perform the same function. 

tolerance—a state of immune 

nonresponsiveness to a particular antigen or 

group of antigens. 

tonsils and adenoids—prominent oval masses 

of lymphoid tissues on either side of the throat. 

toxin—an agent produced in plants and 

bacteria, normally very damaging to cells. 

53 
vaccine—a preparation that stimulates an 

immune response that can prevent an infection 

or create resistance to an infection. Vaccines do 

not cause disease. 

vein—a blood vessel that carries blood to the 

heart from the body tissues. 

virus—a particle composed of a piece of 

genetic material—RNA or DNA—surrounded 

by a protein coat. Viruses can reproduce only 

in living cells. 

54 

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES 

NATIONAL INSTITUTES OF HEALTH 

National Institute of Allergy and Infectious Diseases 

NIH Publication No. 07-5423 

September 2007 

www.niaid.nih.gov