What determines the antibody allotype?

The determination of antibody allotype is primarily associated with the constant regions of heavy and light chains. Allotypes are genetically inherited variations in the constant region of immunoglobulin molecules; these are specific to the isotypes of antibodies and can vary among individuals within a population. Each allotype is defined by specific amino acid substitutions in the constant region, which do not affect the antibody's ability to bind to antigens but can influence the antigen's recognition by the immune system.

The variable regions of heavy and light chains are responsible for the specificity of the antibody to bind particular antigens, which is crucial for its function but does not define allotypic differences. The Fab region, which includes parts of the variable regions, is involved in antigen binding and is not related to allotype variations. The Fc region, while important for interactions with immune effector cells and mediating functions such as complement activation, is similarly not involved in the inheritance of allotypes. Therefore, the constant regions hold the key to determining antibody allotypes, making the correct answer about the constant regions appropriate in this context.