Hereditary deficiency of the second component of complement (C'2) in man.

The serum complement system is composed of at least eight or nine interacting proteins (1, 2). The complete system is required for the reactions of immune cytolysis, immune hemolysis, bacteriolysis, and the lysis of certain tumor cells (1, 3-7). There are other complement dependent systems that do not require all nine components. The reactions of immune adherence and erythrophagocytosis require the first four reacting components of the complement system (2, 8-10). The generation of a chemotactic factor for polymorphonuclear leukocytes requires the activity of the first six reacting complement components (11, 12). Knowledge of the involvement of the serum complement system in these noncytolytic reactions has been acquired only recently and has greatly broadened our concepts of the possible role of the complement system in normal and morbid processes. In addition to these experimental findings, the occurrence of a lowered serum complement level in various acquired pathological states has been well documented (13-17), and the precise cause of the decreased hemolytic activity has been established in at least one human inborn abnormality, hereditary angioneurotic edema (HANE) (18).