Chapter

The Third Component of Complement

Volume 153 of the series Current Topics in Microbiology and Immunology pp 147-162

Factor H

  • D. P. VikAffiliated withDepartment of Immunology, Scripps Clinic and Research Foundation
  • , P. MuñOz-CánovesAffiliated withDepartment of Immunology, Scripps Clinic and Research Foundation
  • , D. D. ChaplinAffiliated withDepartment of Internal Medicine and the Howard Hughes Medical Institute, Washington University School of Medicine
  • , B. F. TackAffiliated withDepartment of Immunology, Scripps Clinic and Research Foundation

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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).