The serum of people with blood group O contains not only classical anti-A and anti-B, but alsoanti-A,B antibodies, i.e., those whose Fab region equally recognizes both the A and B blood group antigens.The common epitope of the A and B antigens, recognized by these antibodies, is located strictly on one sideof the tetrasaccharide molecule, which was shown by studying their complex with anti-A,B antibodies usingthe Saturation Transfer Difference (STD NMR) spectroscopy.