ABSTRACT
Adaptive immune responses evolved to protect vertebrate species against the world of microorganisms. However, anything discerned as ‘nonself’ may become a target of such responses, which can also be directed at molecular differences between individuals within a species. An antibody directed against an antigenic determinant present in some members of a species but not others is said to show polymorphic variation, and antibodies directed against such determinants are called alloantibodies. Perhaps the best known alloantibodies are those that are used to determine our blood groups. Individuals who have red cells of type A have alloantibodies that react with the red blood cells of individuals who are type B and vice versa. Individuals who have red blood cells of type AB have no alloantibodies and those with type O red blood cells have antibodies to both A and B red blood cells (Fig. 45.1). These alloantibodies arise from the fact that the capsules of Gramnegative bacteria, which inhabit our gut, bear antigens that stimulate antibodies that cross-react with the carbohydrate antigens of the ABO blood groups.
