Glycophorin A, the major sialoglycoprotein of human erythrocytes and carrier of MN blood group antigens, is a good model to study immune response against glycopeptide antigens. Molecular analysis of several monoclonal antibodies recognizing GPA revealed that the immune response against N, but not M, antigen is restricted. It was shown that the recombinant Fab fragments expressed in the phage display system may provide novel reagents with application in immunohematology. The new recombinant Fab fragments revealed increased affinity toward GPA. Introduction of dimer-inducing peptides causes creation of bivalent Fab fragments revealing increased avidity.