RESUMEN
C4 sucrose/low ionic strength (LIS)-coated red blood cells (RBC) are excellent for the detection of the previously 'nebulous' antibodies, anti-Chido and anti-Rodgers, as well as for serum/plasma typing of these antigens by an inhibition technique. By enzyme treatment of such cells, it is confirmed that the Ch and Rg antigens reside on the C4d part of the C4 molecule. Freshly taken RBC from normal individuals were examined with a sensitive Auto-Analyzer technique with anti-Chido, anti-C4 and anti-C3 sera. All normal RBC were shown to have C4d and C3d components on their surface. The technique was also very sensitive for the detection of the Ch antigen, which was detected on the RBC of all Chido-positive individuals, and which did not show great variation in strength by this method. The mode of in vivo C4 fixation on normal RBC seems to be different from the fixation in LIS or by RBC antibody-mediated activation.