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The Complement System:An Overview

  • B. Paul Morgan
Protocol
Part of the Methods in Molecular Biology book series (MIMB, volume 150)

Abstract

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 (C1qr2s2)

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.

Keywords

Systemic Lupus Erythematosus Alternative Pathway Lectin Pathway Immune Complex Deposit Meningococcal Meningitis 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Humana Press Inc., Totowa, NJ 2000

Authors and Affiliations

  • B. Paul Morgan
    • 1
  1. 1.Department of Medical BiochemistryUniversity of Wales College of Medicine

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