The immune system distinguishes between self and nonself through an elaborate recognition process. Prior to birth the body inventories the proteins and various other large molecules present (self) and removes T cells specific for self-determinants. Subsequently self substances can be distinguished from nonself substances and lymphocytes can produce specific immunologic reactions against the latter, leading to their removal.
Substances, such as proteins, nucleoproteins, polysaccharides, and some glycolipids that elicit an immune response and react with the products of that response are called antigens (antibody generator). Most antigens are large, complex molecules with a molecular weight generally greater than about 10,000. The ability of a molecule to function as an antigen depends on its size and structural complexity.
Each antigen can have several antigenic determinant sites or epitopes. Epitopes are the regions or sites in the antigen that bind to the antigen-binding site of a specific antibody or with a T cell receptor. Antibodies are formed most readily in response to determinants that project from the foreign molecule or to terminal residues of a specific polymer chain. Chemically determinants include sugars, organic acids and bases, amino acid side chains, hydrcarbon and aromatic groups.
The number of antigenic determinant sites on the surfaces of an antigen is its valence. The valence determines the number of antibody molecules that can combine with the antigen at one time. If one determinant site is present, the antigen is monovalent. Most antigens, however have more that one determinant site or more than one copy of the same epitope, and are termed multivalent. Multivalent antigens generally elicit a stronger immune response than do monovalent antigens.
Many small molecules are not antigenic themselves but can become antigenic if they bond to larger carrier molecules such as a protein. They cannot stimulate antibody formation or T cell responses themselves but can react with antibodies once formed. Such small molecules are termed haptens. When lymphocytes are stimulated by the combined molecule they can react they can react either to hapten or the large carrier molecule. Conjugation of a hapten to a carrier protein makes the haptem immunogenic because the carrier protein can be processed and presented to specific T cells. As a result both hapten specific and carrier specific antibodies can be made. One example of hapten is penicillin. By itself penicillin is not antigenic. However, when it combines with certain serum proteins of sensitive individuals, the resulting molecule does initiate a severe and sometimes fatal allergic immune reaction.
Certain antigens provoke such a drastic immune response that they are termed superantigens. Superantigens are bacterial proteins. These superantigens nonspecifically stimulate T cells to proliferate by interacting with both class II major histocompatibility complex products on antigen presenting cells and the T cell receptor. Good example of superantigens are the staphylococcal enterotoxins that cause food poisoning and the toxin that causes toxic shoxk syndrome. Superantigens cause symptoms by stimulating the release of massive quantities of cytokines from T cells and should be considered possible chronic associates in such diseases as rheumatic fever, arthritis, Kawasaki syndrome, atopic dermatitis, and one type of psoriasis.
