^ Complement - DID Dictionary

Oct 11, 2014 12:30 PM

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Complement: A term originally used to refer to the heat-labile factor in serum that causes

immune cytolysis, the lysis of antibody-coated cells, and now referring to the entire

functionally related system comprising at least 20 distinct serum proteins that is the effector

not only of immune cytolysis but also of other biologic functions. Complement activation

occurs by two different sequences, the classic and alternative pathways. The proteins of the

classic pathway are termed 'components of complement' and are designated by the symbols

C1 through C9. C1 is a calcium-dependent complex of three distinct proteins C1q, C1r and

C1s. The proteins of the alternative pathway (collectively referred to as the properdin

system) and complement regulatory proteins are known by semisystematic or trivial names.

Fragments resulting from proteolytic cleavage of complement proteins are designated with

lower-case letter suffixes, e.g., C3a. Inactivated fragments may be designated with the suffix

'i', e.g. C3bi. Activated components or complexes with biological activity are designated by a

bar over the symbol e.g. C1 or C4b,2a. The classic pathway is activated by the binding of C1

to classic pathway activators, primarily antigen-antibody complexes containing IgM, IgG1,

IgG3; C1q binds to a single IgM molecule or two adjacent IgG molecules. The alternative

pathway can be activated by IgA immune complexes and also by nonimmunologic materials

including bacterial endotoxins, microbial polysaccharides, and cell walls. Activation of the

classic pathway triggers an enzymatic cascade involving C1, C4, C2 and C3; activation of the

alternative pathway triggers a cascade involving C3 and factors B, D and P. Both result in

the cleavage of C5 and the formation of the membrane attack complex. Complement

activation also results in the formation of many biologically active complement fragments

that act as anaphylatoxins, opsonins, or chemotactic factors. [EU]

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