When primary immunity is boosted not by the homologous but by a crossreacting vaccine, the newly formed antibodies react better with the primary antigen than with the antigen actually eliciting the response. This phenomenon bears the name of Original Antigenic Sin (1). It is shown that the number of antibody molecules produced against the original and the vaccinating antigen is the same; that each of these molecules is capable of reacting with both antigens; that the activity of an antiserum can be completely absorbed with either antigen; that both residual and adsorbed-dissociated fractions of antibody exhibit the same relative affinities towards the two antigens as did the native serum; that, unlike standard primary and secondary responses, the population of antibody molecules characterizing the Original Antigenic Sin is homogeneous; that each molecule has a lower equilibrium constant (i.e. higher avidity) against the original antigen than against the antigen stimulating the present response; and that all equilibrium constants are typical of secondary antibody. It is concluded that the Original Antigenic Sin is a partial anamnestic response, a related antigen stimulating that sector only of the originally primed cells which is destined to produce cross-reacting antibody. A hypothesis is developed according to which the basic difference between primary and secondary reactivity rests on the presence of a trapping mechanism that allows anamnestic production of antibody against lower doses of the homologous antigen. Such a mechanism is capable of cross-trapping related antigens, thus preventing a standard primary response and allowing manifestations of Original Antigenic Sin.