The complement components coded in the major histocompatibility complexes and their biological activities.

The complement system has two pathways of activity, both dependent on the sequential conversion of proteolytic zymogens to active proteases leading to a common lytic complex and both with control proteins which inhibit or inactivate different steps in the cascade. Three of the components--C2, factor B and C4--are coded by closely linked genes in the MHC of man and mouse and have been placed relative to each other. The genes are polymorphic, particularly C4, with variable numbers of loci as well as many mutant forms. Some alleles of C4 show strikingly different reactivities in their haemolytic activity and this may be relevant to the association of susceptibility to autoimmune diseases with particular haplotypes in this section of HLA.