Antibody Structure and Function

OBJECTIVES 1. The structure of an antibody is related to its function. a. Studies by Tiselius and Kabat and later by Edelman, Porter, and Nisonoff determined the basic structural components of antibodies. 1) Initially, antibodies were identified only by their electrical charge. 2) Fragmentation studies showed that antibodies are made up of two identical light chains and two identical heavy chains. b. Further studies explained the fine structure of antibodies—the variable and constant regions on the chains. c. The antibody’s antigenic determinants—called isotypes, allotypes, and idiotypes—determine the variability in antibody structure. 1) Isotypes are variants present in all members of a species. 2) Allotypes are variants caused by intraspecies genetic differences. 3) Idiotypes are variants caused by structural heterogeneity in the antibody V regions. 2. There are five classes of antibodies based on the structure of their heavy-chain C domains. a. All antibody classes have either l or k light chains. b. IgG is the major antibody in the blood, but it is able to enter tissue spaces and coat antigens, speeding antigen uptake. c. IgA concentrates in body fluids to guard the entrances of the body. d. IgM is the largest antibody; it tends to remain in the blood, where it can lead to efficient killing of bacteria. e. IgD remains membrane-bound and somehow regulates the cell’s activation. f. IgE is found in trace amounts in the blood, but it still triggers allergies. 3. The biological effector functions of antibodies are mediated by the C domains. 4. Monoclonal antibodies are pure antibodies with single antigenic determinant specificities. a. Monoclonal antibody development improves on nature. b. Special procedures are needed to screen for and isolate monoclonal antibodies. c. Monoclonal antibodies have many applications.