The HIR2 (GA-R2) antibody recognizes an epitope common to the N-terminal region of human CD235a (glycophorin A) and CD235b (glycophorin B), homologous type I sialoglycoproteins of the erythrocyte (red blood cell) membrane that bear the antigenic determinants for the MN and Ss blood groups. CD235ab is expressed on early- and late-stage erythroblasts, mature erythrocytes, and erythroid cell lines such as K562 and HEL, but not on other cell types. CD235a is abundantly expressed whereas CD235b is a relatively minor membrane component. The proteins are believed to provide a large mucin-like surface to erythrocytes that acts to minimize aggregation in the circulation. The HIR2 antibody binds with higher affinity to CD235a than CD235b, and agglutinates untreated erythrocytes.