TRANSFUSION MEDICINE Normal Human Serum Contains Natural Antibodies Reactive With Autologous ABO Blood Group Antigens

It is widely accepted that the serum of healthy individuals contains natural antibodies only against those blood group A or B antigens that are not expressed on the individual’s red blood cells. The mechanisms involved in tolerance to autolo-gous blood group antigens remain unclear. In the present study, we show that IgM and IgG antibodies reactive with autologous blood group antigens are present in the immuno-globulin fraction of normal human serum. Natural IgG anti-A antibodies purified by affinity chromatography from IgG of individuals of blood group A exhibited an affinity for A trisaccharide antigen in the micromolar range and aggluti-nated A red cells at sixfold higher concentrations than those required for agglutination with affinity-purified anti-A IgG of individuals of blood group B. Whereas autoantibodies reac-tive with self A and B antigens are readily detected i