**Immunology** is the study of the **immune system**. The immune system is the parts of the body which work against infection and parasitism by other living things. Immunology deals with the working of the immune system in health and diseases, and with malfunctions of the immune system.

An immune system is present in all plants and animals. We know this because biologists have found genes coding for toll-like receptors in many different metazoans. These toll-like receptors can recognise bacteria as 'foreign', and are the starting-point for immune reactions. The type of immunity which is triggered by the toll-like receptors is called innate immunity. This is because it is entirely inherited in our genome, and is fully working as soon as our tissues and organs are properly developed.

Vertebrates, *and only vertebrates*, have a second type of immunity. This is called adaptive immunity, because it 'remembers' previous infections. Then, if the same infection occurs again, the reaction is much stronger and faster. This immunological memory "confers a tremendous survival advantage" and with it vertebrates "can survive over a long lifetime in a pathogen-filled environment".

Types of immunity in vertebrates
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See the main article: Immune system
### Innate immune response

See the main article: Innate immune system
The innate immune system is usually means all of the cells and systems that does not have to be exposed to a particular pathogen before they can work.

Innate immunity starts with the skin, which is an excellent barrier to infection.

### Adaptive immune response

See the main article: Adaptive immune system
The adaptive immune system includes cells and systems that do require previous exposure to a pathogen. It explains the unique ability of the mammalian immune system to remember previous infections and mount a rapid and robust reaction to secondary infections. This **immunological memory** is due to the biology of T-cells and B-cells.

Other aspects of immunity
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Vaccines boost the acquired immune system by offering weak forms of infection that the body can fight off. The system remembers how to do it again when a stronger infection happens. If the vaccine works, the body can then fight off a serious infection.

The distribution of vaccines and other immune system affecting cures can be considered another level of acquired immune system, one governed by access to vaccination and medicine in general. The intersection of this with the spread of disease (as studied in epidemiology) is part of the field of public health.

### Errors and weaknesses

Errors of the immune system may cause damage. In autoimmune diseases, the body attacks parts of itself because the system mistakes some parts of the body as 'foreign'. Some kinds of arthritis are caused this way.

Sometimes serious pathogens slip in because their surface is disguised as something the host cell walls can accept. That is how viruses work. Once inside a cell, their genetic material controls the cell. Infections like HIV get in this way, and then attack cells which are the basis of the immune system. Artificial means are often used to restore immune system function in an HIV-challenged body, and prevent the onset of AIDS. This is one of the most complex issues in immunology as it involves every level of that system. This research during the 1980s and 1990s radically changed the view of the human immune system and its functions and integration in the human body.

History of immunology
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Immunology is a science that examines the structure and function of the immune system. It originates from medicine and early studies on the causes of immunity to disease. The earliest known mention of immunity was during the plague of Athens in 430 BC. Thucydides (460–395 BC) noted that people who had recovered from a previous bout of some diseases could nurse the sick without contracting the illness a second time.

In the 18th century, Pierre-Louis Moreau de Maupertuis made experiments with scorpion venom and observed that certain dogs and mice were immune to this venom. This and other observations of acquired immunity led to Louis Pasteur (1822–1895) developing vaccination and the germ theory of disease. Pasteur's theory was in direct opposition to contemporary theories of disease, such as the miasma theory. It was not until the proofs Robert Koch (1843–1910) published in 1891 (for which he was awarded a Nobel Prize in 1905) that microorganisms were confirmed as the cause of infectious disease. Viruses were confirmed as human pathogens in 1901, when the yellow fever virus was discovered by Walter Reed (1851–1902).

Immunology made a great advance towards the end of the 19th century, through rapid developments, in the study of humoral immunity and cellular immunity. Particularly important was the work of Paul Ehrlich (1854–1915), who proposed the side-chain theory to explain the specificity of the antigen-antibody reaction. The Nobel Prize for 1908 was jointly awarded to Ehrlich and the founder of cellular immunology, Ilya Mechnikov (1845–1916).

The simplest form of immunity is the DNA restriction system in bacteria that prevents infection by bacteriophages.

Related pages
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* Lymphatic system
* White blood cell