The Nature of Antigens

The term antigen was initially coined in reference to compounds that elicited the production of antibodies; it is derived from the descriptive expression antibody generator. The compounds observed to induce the antibody response are recognized as being foreign to the host by the adaptive immune system. They

16.3 The Nature of Antigens 397

include an enormous variety of materials, from invading microbes and their various products to plant pollens. Today, the term antigen is used more broadly to describe any molecule that reacts specifically with an antibody or a lymphocyte; it does not necessarily imply that the molecule can induce an immune response. When referring specifically to an antigen that elicits an immune response in a given situation, the more restrictive term immunogen may be used. The distinction between the terms antigen and immunogen helps clarify discussions in which a normal protein from host "A" elicits an immune response when transplanted into host "B"; the protein is an antigen because it can react with an antibody or lymphocyte, but it is an immunogen only for host "B," not for host "A."

Various antigens differ in their effectiveness in stimulating an immune response. Proteins and polysaccharides, for example, generally induce a strong response, whereas lipids and nucleic acids often do not. The terms antigenic and immunogenic are used interchangeably to describe the relative ability of an antigen to elicit an immune response. Substances with a molecular weight of less than 10,000 daltons are generally not immunogenic.

Although antigens are generally large molecules, the adaptive immune response directs its recognition to discrete regions of the molecule known as antigenic determinants or epitopes (figure 16.3). Some epitopes are stretches of 10 or so amino acids, whereas others are three-dimensional shapes such as a protrusion in a globular molecule. A bacterial cell usually has many macro-molecules on its surface, each with a number of different epi-topes, so that the entire cell has a multitude of various epitopes.

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