Difference in the biological properties of the two forms of the fourth component of human complement (C4).

The two forms of human C4 were compared in their haemolytic activity and in their capacity to mediate inhibition of immune complex precipitation in human C4 deficient sera. Whereas haemolysis by C4B was 3.2 fold more efficient than by C4A, C4A was 1.7 fold more efficient at inhibiting immune precipitation than C4B. Thus the biological properties of the two forms of human C4 are different and it is suggested that C4A is mainly involved in immune complex clearance reactions.