Relationship of the Anticomplement Coombs Test to the Classes of Immunoglobulins1

Summary The agglutination in saline and the enhancement by anti‐IgG and anti‐C′ Coombs sera of human blood‐group antibodies fractionated by density‐gradient ultracentrifugation and anion‐exchange chromatography, as well as the effect of 0.1M penicillamine on these fractions, have been studied. All of the IgM antibodies agglutinated cells after exposure to C′ and anti‐C′ but not after exposure to anti‐IgG Coombs serum. All IgG antibodies were strongly agglutinated by anti‐IgG, and some (including two unusual anti‐Rh sera) were also agglutinated by anti‐C′ after exposure to C′. A complement‐fixing IgG Donath‐Landsteiner antibody was found to be capable of attaching to erythrocytes in the cold in the absence of complement. If complement has been fixed, the antibody can be subsequently eluted by warming without affecting the attachment of β1c‐globulin. No enhancement of IgM antibody titers could be produced by anti‐IgM Coombs sera unless the Coombs sera also contained anti‐C′. An unidentified anti‐A antibody, approximately 7S, agglutinated more strongly with anti‐C′ than with anti‐IgG Coombs serum. The fixation of complement by anti‐blood‐group antibodies depends on both immunoglobulin class and nature of the antigen. The number and proximity of antigenic sites on the erythrocyte is probably one major factor in determining whether a given IgG antibody can fix complement.