An antigen is any foreign substance, of whatever origin, that is capable of initiating the production of a specific blood protein called an immunoglobulin (e.g., Ig) that will act against it.
The Igs so formed will react specifically with that particular antigen, neutralizing its biological effect. Thus antigens are sometimes referred to as immunogens. However, other antigens possessing a sufficiently similar chemical structure may cross-react with the Ig because antigen–antibody reactions involve close intermolecular binding and depend on a ‘lock and key’ steric fit, similar to the binding of drugs to receptors and enzymes to substrates.
We sometimes make use of this ability to cross-react, e.g. in the old Wasserman test for syphilis antibody, an indicator of past or current infection, but now replaced by an enzyme immunoassay.
The Wasserman reaction does not use Treponema pallidum spirochetes, which are difficult to grow and manipulate, but an artificial antigen prepared from beef hearts that reacts similarly.
Most antigens are proteins. Once these have been recognized by the immune system as ‘nonself’ (foreign), an immune response is initiated.
Microorganisms always express several antigenic groupings (determinants, epitopes) on their surface, so a number of different Igs may be produced against a particular organism. A vast number of non-microbial proteins are capable of stimulating antibody production, including the numerous substances to which allergies are developed.
Macromolecules other than proteins can also lead to the production of antibodies, e.g. lipopolysaccharides. Smaller molecules or ions, e.g. penicillins and heavy metals, may act as antigenic determinants if they combine with ‘self’ (non-antigenic) proteins or cells in an individual, causing the modified protein or cell surface to be recognized as foreign by that person’s immune system.
These small molecules or ions, which cannot themselves elicit the production of an Ig but will react with it when it has been formed in response to the protein–small molecule complex, or a similar cell complex, are termed haptens.
The principal properties of the immunoglobulins
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