The Complement System:An Overview

互联网2014-02-10

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The complement (C) system consists of a group of 12 soluble plasma proteins that interact with one another in two distinct enzymatic activation cascades (the classical and alternative pathways) and in the nonenzymatic assembly of a cytolytic complex (the membrane attack pathway) (Fig. 1 ; Table 1 ). A third activation pathway, termed the lectin pathway, has recently been described ( 1 , 2 ). Control of these enzymatic cascades, essential to prevent rapid consumption of C in vivo, is provided by 10 or more plasma and membrane -bound inhibitory proteins acting at multiple stages of the system. C plays a central role in innate immune defense, which provides a system for the rapid destruction of a wide range of invading microorganisms.

Fig. 1. The complement system and its control. The constituent pathways of the C system and the component proteins are shown. Enzymatic cleavages are represented by thick arrows. The lectin pathway differs from the CP only in that the MBP-MASP complex replaces the Cl complex. Regulators act to inhibit either the enzymes of the activation pathways (activated Cl, C3 convertases, C5 convertases) or assembly of the MAC.

Table 1 The Component Proteins of the Complement System

Component	Structure	Plasma conc (mg/L)
Classical Pathway
C1	Complicated molecule, composed of 3 subunits, C1q (460 kDa), C1r (80 kDa), C1s (80 kDa) in a complex (C1qr ₂ s ₂ )	180
C4	3 chains (α, 97 kDa; β, 75 kDa, γ, 33 kDa); from a single precursor	600
C2	single chain, 102 kDa	20
Alternative Pathway
fB	single chain, 93 kDa	210
fD	single chain, 24 kDa	2
Properdin Common:	oligomers of identical 53 kDa chains	5
C3	2 chains: α, 110 kDa, β, 75 kDa	1300
Terminal Pathway
C5	2 chains: 115 kDa, 75 kDa	70
C6	single chain, 120 kDa	65
C7	single chain, 110 kDa	55
C8	3 chains: α, 65 kDa, β, 65 kDa γ, 22 kDa	55
C9	single chain, 69kDa	60

The proteins that constitute the classical, alternative, and membrane attack pathways are listed together with their approximate concentration in plasma. Modified from: Morgan, B.P. and Harris, C. L. (1999) Complement Regulatory Proteins. Academic, London.

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