It can be concluded that following exposure to a foreign substance, whether it is a bacterium, virus or pollen, the body develops something which neutralises or destroys the substance and which acts specifically with only that substance or one very similar to it. As an example of this, vaccination (vacca from the Latin for cow) involves injection of the cowpox virus which confers immunity against smallpox, the virus of which is similar. It can, however, occasionally result in vaccinia, a human form of cowpox. Edward Jenner first used fluid extracted from the cowpox blisters of a milkmaid to vaccinate a child against smallpox in 1796. His research which culminated in the development of this vaccination arose from the observations of farmers that milkmaids who had suffered from cowpox were immune to smallpox. The term vaccination has come to be used loosely for any immunisation.

The immune system is primarily a defensive system which protects the individual against a variety of foreign substances including micro-organisms, transplanted cells and irritants. The mechanisms involved are relevant to a wide variety of clinical situations, including immunity against infectious disease, the rejection of transplanted organs, compatibility of blood for transfusion, allergic disorders, autoimmune diseases and malignant conditions.

Although the agents conferring immunity, which are antibodies (specialised blood proteins) and components of certain lymphocytes, have been recognised since the end of the nineteenth century, immunity remains a subject of intensive research, and many questions remain unanswered.

An antigen is an agent or substance which can be recognised by the body as 'foreign'. Often it is only one relatively small chemical group of a larger foreign, substance which acts as the antigen, for example a component of the cell wall of a bacterium. Most antigens are proteins, though carbohydrates may act as weak antigens.

The body reacts to antigens by making antibodies, themselves proteins, or special lymphocytes carrying an antibody-like component on their cell surface. These antibodies or lymphocyte components interact chemically with the antigen in a highly specific manner a bit like two pieces of a jigsaw. However, occasionally antibodies or lymphocyte components may 'fit' other similar antigens, in much the same way as a piece of a jigsaw will sometimes work with other bits it wasn't originally intended to.

When the antigen and antibody interact, they bind together to form an antigen/antibody complex, or immune complex, and this binding neutralises or brings about the destruction of the antigen.

A bacterial toxin may, for example, cause disease by binding to a particular type of cell in the body. When an antibody interacts with the toxin it may neutralise its toxicity by, perhaps, binding to the active site of the toxin, that is, the part which binds to the target cell. Alternatively, interaction between antibody and toxin may alter the shape of the toxin so that it no longer has the capacity to bind to cells.

Other ways in which such interaction destroys the antigen include an increased susceptibility of the immune complex to phagocytosis by leukocytes.