Complement (immunologic)

Complement, or the complement system, is a major part of immune response. It involves the interaction of at least 20 proteins, some of which initiate the complement response, amplify or modulate the response, and start a chain of complement protein conversions that result in proteins that attack cell membrates. Ideally, these attacks are against invading cells, but they may attack the healthy cells of the body in an autoimmune disease.

Complements are abbreviated C99 for the complement protein number; there may be suffixes to describe subclasses of a protein. There are a number of uniquely named proteins that have regulatory effects on the process. Some of these have context-dependent abbreviation, such as B for "Factor B" or C3 convertase for the enzyme that converts ordinary C3 to C3a.

Activation
Two main pathways lead to the activation of complement. One is C1 and its subcomponents, which recognize an antigen on a cell wall.

The other starts with complement factor B binding to a cell surface, rather than to a specific antigen.

Biologic activity
{| class="wikitable" Some Effects of Complement Proteins ! Substance ! Effects ! Process/receptors/cells affected * Increase of vascular permeability * Degranulation of mast cells and basophils causing histaminerelease }
 * C3a
 * Smooth muscle contraction
 * Smooth muscle contraction
 * Smooth muscle, basophils, mast cells, platelets, eosinophils
 * C3b
 * Opsonization and making immune complexes soluble, to amplify later phagocytosis
 * C3e
 * Release of leucocytes from blood marrow
 * C3e
 * Release of leucocytes from blood marrow
 * Release of leucocytes from blood marrow