Metabolism of radiolabelled complement proteins in health and disease

Abstract

The complement system consists of some twenty serum proteins that interact with one another in highly specific ways as part of host defence and of the inflammatory reaction, primarily in response to infection and in the clearance of immune complexes¹. The system appears also to participate in the pathophysiology of ‘autoimmune’ diseases such as systemic lupus erythematosus and glomerulonephritis². In the latter disorders, lowering of the serum concentration of individual complement proteins, deposition of complement proteins in affected tissues, particularly renal glomeruli, and the presence in the circulation of complement protein fragments are taken as evidence of complement activation in vivo, probably by immune complexes. The concentration of some complement proteins is also lowered in hepatic diseases. Because many complement proteins are produced primarily in the liver, it has been suggested that failure of the protein-synthesising capacity of this organ may contribute to or be largely responsible for lowered complement in liver diseases. Many, but not all, complement proteins are acute-phase reactants and rise in serum concentration following tissue necrosis or inflammation. Finally, there are inherited deficiency states, mostly rare, for almost all complement proteins. For inherited deficiencies of complement components themselves, it is, by and large, only the affected protein which is markedly decreased in concentration or absent. On the other hand, deficiency states of control proteins produce complex patterns of reductions in the concentrations of certain individual proteins.