Epitope specificity of blood-group-A-reactive murine monoclonal antibodies.

Monoclonal antibodies (MABs) towards blood groups A manifested three broad patterns of binding distinguished on the basis of affinities for A1 or A2B phenotype red blood cells and blood group substances. Competitive inhibition of binding of radiolabelled MABs by other unlabelled antibodies and absorption studies with synthetic haptens suggested that GpA specificity was expressed as two topographically related structures, one being the terminal GpA trisaccharide and the second probably involving part of the oligosaccharide backbone. The enhancement of binding of an extremely low-affinity antibody by higher affinity antibodies recognising either epitope indicated a topographic, possibly conformational relationship between these epitopes. It is suggested that only those antibodies that recognise terminal GpA trisaccharides can agglutinate weak A2B cells.