Natural IgM antibodies to autologous A and B blood group antigens in normal human serum. Upper panels: reactivity of IgM in whole serum of individuals of blood group A (□), B (◊), O (▵), and AB (○) with immobilized A (left panel) and B (right panel) antigens, as assessed by ELISA. Lower panels: reactivity of IgM (IgG-depleted serum) of individuals of blood groups A (□), B (◊), O (▵), and AB (○) with A (left panel) and B (right panel) antigens. Each data point represents mean OD ± SD obtained on testing IgG of four individuals in each blood group category.