This traditional description of an antigen is no longer comprehensive enough in the light of current concepts about the immune response.
Some antigen may not induce antibodies but many sensitive specific lymphocytes leading to cell mediated immunity or may cause immunological tolerance.
The word ‘parenteral’ is used in the definition because orally administered antigens are usually denatured by digestive enzymes and their antigenicity destroyed, so that no antibody formation take 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 or cells complementary to that antigen only. The antibody so produced will react only with that particular antigen and with no other, though immunological cross reaction may occur between closely related antigens.
The two attributes of anigenicity are (1) induction of an immune response (immunogenicity), and (2) specific reaction with antibodies or sensitized cells (immunological reactivity). Based on the ability to carry out these two functions, antigens may be classified into different types.
Haptens are substances which are incapable of inducing antibody formation by themselves but can react specifically with antibodies. Haptens become immunogenic on combining with a larger molecule carrier.
The smallest unit of antigenicity is known as the antigenic determinants or epitope. The epitope is that small area on the antigen, usually consisting of four or five aminoacid or monosaccharide residues, possessing a specific chemical structure electrical charge and steric configuration.
The combining area on the antibody molecule, corresponding to the epitope, is called the paratope. Epitopes and paratopes determine the specificity of immunological reactions.
Antigens such as bacteria or viruses carry many different types of epitopes, presenting an antigenic mosaic. The presence of the same or similar epitopes on different antigens accounts for one type of antigenic cross reaction.