Antigens are any substances that are capable, under appropriate conditions, of inducing the formation of antibodies and reacting specifically with the antibodies so produced. They react with both T-cell recognition receptors and with antibodies. These antigenic molecules may have sev-eral antigenic determinants, called epitopes, and each epitope can bind with a specific antibody. Thus, a single antigen can bind to many different antibodies with different binding sites.
Some low-molecular-weight mol-ecules called haptens are unable to evoke an immune response but can react with existing antibodies. These molecules need to be coupled to a carrier molecule to be antigenic.
For some molecules such as drugs, the molecule needs to be conjugated to a car-rier. The carrier may be a host protein. The tertiary structure of the molecule as well as the amino acid sequence is important in determining antigenicity. Certain struc-tures such as lipids and DNA are generally poor antigens.
Most antigens are either thymus-dependent or thymus-independent anti-gens. Thymus-dependent antigens require T-cell participation: Most proteins and foreign red cells are examples of these molecules. Thymus-independent antigens do not require T-cell participation for anti-body production. Instead, they directly stimulate specific B lymphocytes by cross-linking antigen receptors on the surface of B cells. These molecules produce primarily IgM and IgG2 antibodies and do not stimu-late long-lasting memory cells. Most bac-terial polysaccharides (found in bacterial cell walls) fall into this category. Certain polysaccharides, such as LPS (lipopoly-saccharide), not only induce specific B-cell activation but also can act as a polyclonal B-cell stimulant.