Component of complement
Any one of the nine distinct protein units (designated C1 through C9 and distributed in the a, b, and g electrophoretic partitions of normal serum) that effect the immunological activities long associated with complement. C1 is a complex of three subunits: C1q, C1r, and C1s. C1q (overbar indicates active form) activates proenzyme C1r to C1r which activates C1s to C1s (also known as C1 esterase), which converts proenzyme C2 to C2b and produces C4b from C4. C2b combines with C4b to form classical-complement-pathway C3/C5 convertase (also known as C3 convertase, C5 convertase, and C42). This enzyme cleaves C3 to C3a and C3b, and C5 to yield C5a and C5b, as does alternative-complement-pathway C3/C5 convertase (also known as proenzyme factor B, properdin factor b, c3 proactivator, and heat-labile factor). Complement factor i (also known as C3b or C3b/C4b inactivator) inactivates C3b and C4b by a different proteolytic cleavage. Several autosomal recessive disorders have been identified in which one or more of the complement components have been deficient or completely absent.