The significance of erythrocyte antigen site density

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Abstract
The importance of antigen site density has been studied by means of a model passive hemagglutination system using human red cells coupled with sulfanilic acid groups. Relative site numbers were estimated from the covalent linkage of sulfanilic acid-35S to red cell membrane protein and the effective antigen site number was determined with 125I-labeled rabbit IgG antisulfanilic acid. Cells which had fewer than 20,000 antigen sites per cell were not agglutinated. As greater numbers of sulfanilic groups were coupled to the red cells, the agglutination titers increased to maximum values with red fanilic groups were coupled to the red cells, the agglutination titers of purified IgM antibody were 10-20 times greater than IgG antibody when preparations with the same protein concentration were compared, but this difference was not noted when IgG antibody was measured by antiglobulin reactions. These findings emphasize the need to consider differences in antigen site density when comparing blood group systems. They are consistent with the hypothesis that those blood group antigens which have a very low site number will not be detected by IgG antibodies in saline hemagglutination determinations.