Identification of a C4 Subcomponent on C3d-coated erythrocytes

Test erythrocytes (E) used to evaluate anti-complement (C') antiglobulin sera have not been adequately standardized. This report describes a previously unrecognized C4-derived antigen (temporarily called X-Ag) found on E generally believed to be coated only with the C3d subcomponent of C3, X-Ag occurred on all E coated in vitro with C' by low ionic strength-sucrose or cold agglutinin methods and on E from ten of ten patients whose cells had been C' coated in vivo. It was not removed by incubating these cells with trypsin or fresh compatible serum. This antigen was found on "C4-only-coated" red blood cells made with normal or congenitally C2-deficient serum but not on cells similarly prepared with congenitally C4-deficient serum. It was not identified on E coated with C' via the alternate pathway, normal trypsinized cells, nor cells coated only with IgG. Absorption experiments utilizing purified complement components and subcomponents and G200 Sephadex fractions of normal human serum strongly suggest that X-Ag is a subcomponent of C4(C4d). These results show that at least one C' subcomponent other than C3d occures on both in vitro and in vivo C3d-coated erythrocytes and must be taken into account when such cells are used to evaluate antiglobulin reagents.