Origin of the fourth component of complement related Chido and Rodgers blood group antigens.

We have reviewed the relationship between C4 and its related blood group and discussed the mechanisms whereby a fragment of C4 could become attached to erythrocytes (E). We hypothesize that there is chronic fluid-phase activation of C4 by either C1 to form C4b or spontaneous cleavage of the thioester to form iC4. These activated molecules bind to E. Proteolytic degradation of the bound C4b or iC4 would leave a covalently attached fragment of C4 on E and thereby give rise to the Ch and Rg blood group antigens. This system is of further immunopathologic interest since this 'normal' activation or turnover of C4 is closely regulated. In patients deficient in regulatory proteins, this spontaneous or normal turnover of C4 and C3 may initiate a pathologic condition.