An antigen (G. anti-against, genos-genus) is such a macromolecules which, upon introduction into the body, stimulates production of antibodies and react specifically with them. The antigenic macromolecules include almost all proteins, most polysaccharides, nucleoproteins, lipoproteins, glycoproteins, and various small bio-chemicals if they are attached to proteins or polypeptides.
Antigens are characterized by two essential properties:
(i) The ability to cause the production of antibodies (antigenicity or immunogenicity), and
(ii) The ability to enter into an interaction with the corresponding antibodies (antigenic specificity or specific reactivity).
Most of the antigens are “complete”, that is, they possess both of the essential properties, and are thus called complete antigens. A complete antigen molecule, however, consists of a “carrier portion” that is necessary for the stimulation of antibody-production, and a “reactive portion” that reacts chemically with the antibody molecule. A complete antigen molecule usually has molecular weight of 10,000 daltons or more.
Besides, there are partial antigens also which do not have ability to stimulate antibody production as they lack “carrier portion” but, they may react with antibodies because they possess “reactive portion”.
These partial antigens are called haptens and include lipids, complex carbohydrates and other substances. Haptens, when they combine with such a protein that acts as “carrier portion”, become complete antigens (Fig. 41.5).
For instance, some low molecular drugs like penicillin act as haptens because they may combine with high molecular weight proteins in the body and become complete antigenic.
The key feature of an antigen is its area of reactivity, called antigenic-determinant-site, the site where the antigen combines with the antibody. The number of antigenic determinant site on the surface of an antigen is called valence.
If an antigen possesses single antigenic determinant site, it is called monovalent while it is called multivalent, if it consists of more than one antigenic determinant sites on its surface. Generally, multivalent antigens elicit an stronger immune response than monovalent antigens.