FOUR FIGURES
Typically, antibodies are recognized according to the qualities in which they are diverse. For convenience, these are classified according to primary properties and secondary reactivities. The primary properties are : (1) antigenic specificity, which in the most precise sense indicates the particular chemical or spatial configuration with which the antibody combines; (2) avidity for ant.igen, which is measured by the equilibrium constant of the reaction ; and (3) physicochemical properties, such as size, shape, electrophoretic mobility, and amino acid sequence. Secondary reactivities include : (1) aggregating capacity ; (2) complement-fixing capacity; and
(3) biological activity. The last-named property refers to the cytotoxic or allergic responses resulting from the antigenantibody union.
There is little reason to doubt that antibodies differ in all the respects listed. The question at issue is how these various differences relate to one another. Undoubtedly, one of the most important factors influencing all the other properties of an antibody is its antigenic specificity or the properties of the antigen to which it is bound. For example, the nonagglutinating or blocking property of certain Rh antibodies has been attributed to the location of the complementary antigen sufficiently deeply within the red cell surface structure to prevent the antibody molecule from linking two cells (Coombs et al., '51).