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Antigens, or immunogens, are substances or molecules that are foreign to the body but when introduced trigger the production of antibodies by B lymphocytes leading to the ultimate destruction of the invader. They are usually large macromolecules (>10,000 Da) such as proteins, polysaccharides, lipids, and free nucleic acids. Antigens are recognized by specific receptors present on the surface of lymphocytes and by the antibodies or immunoglobulins secreted in response to the antigen. Antigens can take the form of any foreign substance including bacteria, fungi, viruses, protozoa, parasites, and nonmicrobial agents such as plant pollens, insect venom, and transplanted organs.

Multiple epitopes on a complex antigen being recognized by their respective (A, B, C) antibodies.

Antigens possess immunologically active sites called antigenic determinants, or epitopes. These are smaller, discrete components of the antigen that have a unique molecular shape, which can be recognized by and bound to a specific Ig receptor found on the surface of the lymphocyte or by an antigen-binding site of a secreted antibody (Fig. 13.4). It is not unusual for a single antigen to possess several antigenic determinants and, therefore, be capable of stimulating several different T and B lymphocytes. For example, different proteins that comprise the influenza virus may function as unique antigens (A, B, C, H, and N antigens), each of which contains several antigenic determinants. Hundreds of antigenic determinants are found on structures such as the bacterial cell wall.
Low molecular weight molecules (<10,000 Da) may contain antigenic determinants but alone are usually unable to stimulate an immune response. These molecules are known as haptens. When they are complexed with an immunogenic carrier (usually a protein), they function as antigens. Many haptens exist in nature and frequently create problems for humans. Urushiol is a toxin found in the oils on poison ivy that is responsible for initiating an allergic reaction. An allergic response to the antibiotic penicillin is an example of a medically important reaction due to hapten–carrier complexes. The penicillin molecule is very small (350 Da) and usually nonantigenic. However, in susceptible people it can complex with carrier proteins in the body, which are then recognized as “foreign” and capable of initiating an antigen–antibody reaction.