Natural antibodies and immune antibodies of human ABO blood group system

Sera of normal and isoimmunized persons genotype OO and A1O were fractionated to separate the three main immunoglobulin components IgG, IgM, and IgA from each other. The anti-B activity was measured in every fraction and individual serum. The results indicate that the natural anti-B antibodies are of IgM molecular type, contrasting with the immune anti-B antibodies which are IgM, IgG and eventually IgA. The restriction to the IgM class and other data previously reported are discussed in relation to the origin of the natural antibodies of the human ABO blood group system.