The complement proteins play an important role in innate immunity, promoting inflammation and microbial killing. They play a role in the adaptive immune response, as well. Inherited total deficiencies of complement proteins are extremely rare. Table 1 lists more than 40 proteins that comprise the elements of the complement system. Deficiency of the proteins that promote lysis and opsonization is so rare that two papers are able to list all the observed cases to 1991. The exception is mannan-binding lectin, in which deficiency may be commoner. Diseases of regulatory proteins, such as occurs in hereditary angioedema or paroxysmal nocturnal hemoglobinuria, are commoner but still are quite rare. As we learn more about complement proteins and their mechanism of action, we will understand more clearly how these proteins function. Polymorphisms of the proteins exists. Learning how these polymorphisms contribute to the development of disease will be the focus of complement studies in the next decade.
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