Chapter: Essential Clinical Immunology: Basic Components of the Immune System

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Antigens

Antigens are any substances that are capable, under appropriate conditions, of inducing the formation of antibodies and reacting specifically with the antibodies so produced.

ANTIGENS

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.


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