Localisation of Chido and Rodgers determinants to the C4d fragment of human C4

CHIDO (Ch) AND RODGERS (Rg) are blood-group antigens present in plasma and on red cells. Interest in these antigens was stimulated when Ch (ref. 1) and Rg (ref. 2) were found to be genetic markers for the major histocompatibility complex, HLA, and when strong linkage disequilibrium between the Rg-negative phenotype and HLA-A1, B8 homozygosity was noted3. Ch and Rg recently attracted a broad audience when O'Neill et al.4 showed that the Ch and Rg phenotype of plasma was correlated with the electrophoretic polymorphism of human C4, the fourth component of complement, and that C4 purified to homogeneity inhibited anti-Ch and anti-Rg. We have studied the transfer of Ch and Rg from plasma to red cells in conditions in which C4 is transferred. We report here a correlation between the Ch, Rg polymorphism and functional C4, and show that Ch and Rg reside on the C4d fragment of the molecule. We also report that a monkey antiserum to human C4d (ref. 5) is predominantly anti-Ch and anti-Rg.