Expression of A antigens on erythrocytes of weak blood group A subgroups.
Scanning immune electron microscopy using a monoclonal anti-A antibody which reacts with all type A oligosaccharide chains revealed A antigens on less than 5% of Am and Ael cells, some of which showed extremely strong labelling. This explains why Am and Ael cells can absorb significant amounts of anti-A without being agglutinated. A3 may be a heterogeneous subgroup, since A antigens were found on 82 and 58%, respectively, of the cells of 2 A3 individuals. A antigens were found on 75% or more of Ax cells. In many weak A individuals A-positive cells are apparently best detected if an anti-A is used which reacts strongly with other A oligosaccharide chains than type 2. From hyperimmune pregnancy sera Ax, Am and Ael erythrocytes absorbed antibodies which seemed to have other fine specificities than those absorbed by A2 cells. We conclude that weak subgroups of A may deviate from A2 both by number of erythrocytes expressing A antigens and the biochemical nature of the antigens.