An antigen has been defined as any substance

which, when introduced parenterally into the body stimulates the production of an antibody with which it reacts specifically and in an observable manner. This traditional description of an antigen is no longer comprehensive enough in the light of current concepts about the immune response. Some antigens

may not induce antibodies but may sensitise specific lymphocytes leading to cell mediated immunity or may cause immunological tolerance.

The word 'parenteral' (meaning, outside the intestinal tract) is used in the definition because orally administered antigens are usually denatured by digestive enzymes and their antigenicity destroyed, so that no antibody

formation takes place. When given parenterally, antigens do not undergo any such inactivation and can induce antibody production. However, there are exceptions and some antigens can be immunogenic when given orally, such as oral vaccines. The word 'specifically in the definition is important as specificity is the hallmark of all immunological reactions.

An antigen introduced into the body reacts only with those particular immunocytes (B or T lymphocytes) which carry the specific marker for that antigen and which produce an antibody cells complementary to that antigen only. The antibody so produced will react only with that

particulate antigen and with no other though immunological cross-reaction may occur between closely related antigens.

The two attributes of antigenicity are

1. induction of an immune response (immunogenicity),and

2. specific reaction with antibodies or sensitised cells (immunological reactivity).