Immunochemical methods use antigens and antibodies as tools to detect microorganisms. Antigens are foreign substances, usually high-molecular-weight proteins or carbohydrates, that elicit the production of other proteins, called antibodies, in a human or animal host (see Chapter 3). Antibodies attach to the antigens and aid the host in removing the infectious agent (see Chapters 3 and 10). Antigens may be part of the physical structure of the pathogen, such as the bacterial cell wall, or they may be a chemical produced and released by the pathogen, such as an enzyme or toxin; each antigen is also called an epitope. Figure 9-1 shows the multiple epitopes of group A Streptococcus (Streptococcus pyogenes).
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