The immunochemical properties of purified A1 and A2 glycoproteins have been compared to ascertain whether their antigenic determinants differ. Quantitative precipitin and complement-fixation studies using several anti-A sera as well as purified γG anti-A antibodies clearly showed a specificity difference. This was also supported by absorption studies: A2 substance specifically removed antibodies reacting with A2 substance leaving anti-A1 activity. A1 substance was more effective than A2 substance in dissolving an A1 anti-A1-specific precipitate. Purified γM anti-A hemolyzed A1 cells more readily than A2 cells. Inhibition studies using mono- and difucosyl type 2 A-active oligosaccharides showed that type 2 difucosyl receptors are present in A2 substance. The structural basis for the specificity difference between A1 and A2 would appear to be that A2 substances lack type 1 A determinants; this would account for the observed higher H and Leb activity in A2 substances.

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