Structural polymorphism of the fourth component of human complement.

The fourth component of human complement (C4) in 102 individual plasma samples has been examined by the technique of antigen-antibody crossed electrophoresis (AACE). Electrophoretic heterogeneity of C4 was manifested by the repeated occurrence of seven different precipitin patterns. These patterns were formed by varying combinations of three subtypes of C4, differing in electrophoretic mobility. The subtypes were designated C, A, and A(1), in order of increasing electrophoretic mobility toward the anode. The evidence that the observed electrophoretic heterogeneity of the C4 molecule represents structural polymorphism rests on five points: the pattern obtained from the plasma of a given individual was reproducible in different runs and with different bleedings; all seven patterns could be demonstrated on the same electrophoretic run; C4 of a given subtype retained its characteristic mobility after purification, when run alone or mixed with plasma containing C4 of other subtypes; the subtypes A(1) and C comprising pattern 6 could be separated chromatographically as well as electrophoretically; and the characteristic relative mobilities of different C4 subtypes, in plasma or after purification, were retained even after the rather large shift in mobility associated with conversion to C4i. The ratio of C4 hemolytic activity to protein concentration varied according to the subtype composition of individual samples, with highest ratios occurring with patterns composed of subtype C alone, intermediate values with patterns consisting of A and C, and lower values occurring with patterns containing subtype A alone. Although the mechanism of inheritance of this polymorphism is not yet clear, the data suggest that subtypes A and A(1) are inherited as autosomal codominant characteristics, independent of the inheritance of subtype C.