Factor H

Abstract

Complement component factor H (H) is a 155-kDa plasma protein. It was first observed by NILSSON and MüLLER-EBERHARD (1965) as a minor component in C3 preparations, and its function was not known. They termed this protein β₁ H globulin because of its β-electrophoretic mobility in agarose gels. The first function for this protein was recognized by WHALEY and RUDDY (1976a). They isolated a protein from human serum based on its ability to accelerate the activity of C3b inactivator (now termed factor I) and found that this protein was biochemically and immunologically identical to β₁ H globulin. At the time, it was thought that I alone cleaved C3b into two fragments, C3c and C3d. Addition of H synergistically enhanced the activity of I-mediated cleavage of C3b. H not only enhanced the activity of I (termed its cofactor activity) but was also able to accelerate the decay of the C3 convertases of the alternative pathway of complement activation C3b, Bb or C3b, Bb, P (Fig. 1) and to inhibit the formation of these convertases (WHALEY and RUDDY 1976 a).