Glycophorin A-deficient red cells may have a weak expression of C4-bound Ch and Rg antigens

The blood group antigens Ch and Rg are polymorphisms of C4d. Antigen-positive red blood cells (RBCs) treated with proteases type as Ch-, Rg-. Although RBCs treated with sialidase may type Ch+ Rg+, they cannot be coated with C4 by the 10 percent sucrose method. Since studies of complement binding have shown that glycophorin A (GPA) is an important component for the uptake of C4 by RBCs, we tested all available GPA-deficient RBCs for their Ch and Rg status. Using eluates of human anti-Ch and anti-Rg, and monoclonal anti-Rg, we found that the Ch antigen was only weakly expressed on these RBCs, while Rg expression was variable. Our results imply that in the absence of GPA, C4 binds in vivo to a component or components other than GPA on RBCs.