Antigenic Determinants of C3 and C4 Complement Components on Washed Erythrocytes from Normal Persons

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Abstract
Well-washed erythrocytes [RBC] from normal persons were agglutinated by antisera to C3 [complement component 3], C3d and C4, and this agglutination was specifically inhibited by the corresponding C3 or C4 protein. C3 and C4 antigenic determinants were present on the RBC of freshly shed blood promptly anticoagulated with EDTA, heparin, ACD [acid-citrate-dextrose], or CPD [citrate-phosphate-dextrose] and no significant changes in degree of agglutination were observed on storage of EDTA or CPD blood for 2 wk at 4.degree. C. Marked differences in degree of agglutination by anti-C3, anti-C3d and anti-C4 were observed when RBC of 16 normal persons were assayed, and significant correlations were obtained when the quantitative results with any 2 antisera were compared. Anti-C3c did not agglutinate RBC from normal persons, suggesting that the C3 antigens detected on normal cells are carried by the C3d fragment. To avoid significant agglutination of the RBC from some normal persons, very dilute preparations of anti-C3, -C3d and -C4 were used for instrumented diagnostic direct antiglobulin tests. Stronger reagents could be used for indirect antiglobulin tests when the result of a suitable control could be subtracted.