Landsteiner, knowing that none of his subjects had been immunized, realized that “natural” antibodies must develop which are directed against antigens not present on the red cells. Individuals with “A” antigens on their red cells had sera containing “Anti-B” antibody. Individuals with “B” antigens had sera containing “Anti-A.” “AB” individuals had sera with no ABO antibodies present and “O” individuals’ sera contained “Anti-A” and “Anti-B.” Sera from group O individuals may contain a separate antibody, “Anti-A,B.” Anti-A,B possesses serologic activity not found in mixtures of Anti-A and Anti-B. Anti-A,B sera will agglutinate A, B, and AB cells. It is particularly useful in detecting weak A and B antigens. See the table on the next page.