The Complement System: A Brief Overview

The term complement has been used to refer to the heat-labile component of blood that was able to lyse bacteria. As a result of extensive research on it, the other ways in which it contributes to the host defense system is known. Complement part of the immune system can opsonize bacteria for enhanced phagocytosis. It can also recruit various cells such as the polymorphonuclear lymphocytes and macrophages to the site of infection. It also participates in the regulation of the antibody responses and can aid in the clearance of immune complexes and apoptotic cells.

The compliment can have destructive effects on the host as it contributes to inflation and tissue damage as well as anaphylactic reactions. It is composed of 30 different serum proteins that are secreted by different cells including the hepatocytes, macrophages and epithelial cells.

Some of these proteins bind to the antibodies or to membrane components of the cells while others are proenzymes that when activated cleave one or more other complement proteins to produce products that activate cells or increase vascular permeability or opsonize bacteria.

The compliment protein includes:

  • C1 (q,r,s) to C9
  • C1 inhibitor (serpin)
  • Complement Receptor 1 (CR1)
  • C4 Binding protein
  • Protein S ( Vitronectin)
  • Decay Accelerating Protein (DAP)
  • Factors B,D,H,I,P(Properdin)
  • Mannose Binding Lectin (MBL)
  • MBL associated Serine Protease 1 and 2

There is a starting point for activating the complement protein for subsequent activations until the Membrane Attack Complex (MAC) is formed. When enzymatically cleaved, This results in a small (a) and larger (b) fragment. The latter attaches to the activation complex membrane and the former is released to the microenvironment. The fragments are named universally as a and b for the smaller section and the larger sections respectively except for C2 where the largest fragment is a and the smaller fragment b