An antibody identifies its corresponding antigen by one or more regions on the antigen known as the epitopes, which are also called antigenic determinants. The epitope must be the right size, shape, and chemical structure for the antibody to bind to the epitope and then proceed to disable or destroy the antigen.
Antigens tend to have a molecular weight of 10,000 or more yet some foreign substances might have a lower molecular weight and are not antigens. They are called haptens and must attach themselves to a large carrier molecule in order to become antigenic. Antibodies only attack the hapten and not the carrier molecule.
Penicillin is a common hapten. Penicillin does not have an antigen effect in most people. However, when penicillin attaches to serum proteins an allergic reaction results in some people. These people are said to be allergic to penicillin. An allergic reaction is a typical immune response.
Antigens can be proteins, large polysaccharides, lipids, or nucleic acids. However, antigens that are lipids and nucleic acids must be combined with proteins and polysaccharides; otherwise they are not antigens.
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