Deficiency of the first component of human complement.

C1 deficiency results from an absence or lowering of the level of one or more of the proteins C1q, C1r and C1s, which are the subcomponents of the C1 complex of the classical pathway of the serum complement system. The major clinical pattern shown in such deficiency states is an inability to deal effectively with immune complexes, resulting in the typical symptoms associated with immune-complex-related diseases and a great susceptibility to recurrent bacterial infections. Both acquired and genetic deficiencies of the C1 subcomponents have been reported; the possible genetic deficiencies appear quite rare, with only 14 reports of C1q deficiency (involving 24 people) and six reports of C1r/C1s deficiency (involving 11 people) appearing in the literature to date.