Complement: function and clinical relevance.

There is a remarkable array of proteins participating in the complement cascade, regulating the activation of the system, or recognizing a fragment of a component as a biologic signal. The classical pathway of complement activation depends on antigen-antibody interaction and is important as an effector arm of acquired humoral immunity to microorganisms. The alternative pathway functions as a form of innate humoral immunity by attacking membranes not having the characteristics of self-membrane. In addition, the alternative path provides amplification after triggering by either path. Absence or dysfunction of many of the components is associated with autoimmune or immunodeficiency disease. Absence of the inhibitor C1INH is associated with the unique syndrome of hereditary angioedema.