Effector Functions of Complement
المؤلف:
APURBA S. SASTRY , SANDHYA BHAT
المصدر:
Essentials Of Medical Microbiology 2021
الجزء والصفحة:
3rd edition , p166-167
2025-08-25
563
The membrane attack complex (MAC) and other complement by-products produced during the activation of complement pathways augment the immune response in many ways; which are collectively called as the effector functions of complement products. The functions are as follows:
1. Target cell lysis by MAC: As already explained, the MAC makes pores or channels in the target cell membrane; thereby allows the free passage of various ions and water into the cell leading to cell swelling, lysis and death. Bacteria, enveloped viruses, damaged cells, tumor cells, etc. are killed by this mechanism, commonly referred to as complement-mediated cell lysis (Fig. 1A)
2. Inflammatory response: Complement by-products such as C3a, C4a and C5a are called anaphylatoxins. They bind to surface receptors of mast cells and induce their degranulation leading to release of histamine and other inflammatory mediators. They cause vasoconstriction, and increased vascular permeability (Fig. 1B)
Fig. A and B: A. Complement mediated cell lysis; B. Activation of inflammatory response.
3. Opsonization: C3b and C4b act as major opsonins that coat the immune complexes and particulate antigens. Phagocytic cells express complement receptors (CR1, CR3 and CR4) for complement components (C3b, C4b), and are able to bind to complement coated antigens and enhance phagocytosis (Fig 2). C5a augments this process by enhancing the CR1 expression on phagocytes by 10 folds
Fig2. Complement-mediated opsonization.
4. Removing the immune complexes from blood: C3b plays an important role in removing immune complexes from the blood. C3b bound immune complexes are recognized by complement receptor CR1 present on RBCs. Immune complexes bound to RBCs are taken to liver and spleen where they are phagocytosed after being separated from the RBCs (Fig.3)
Fig3. Clearance of immune complexes.
5. Viral neutralization: Complements play a crucial role in neutralization of the viruses; which occurs by many ways—
- Complements coated on virus surfaces neutralize the viral infectivity by blocking their attachment sites
- C3b mediated opsonization of viral particles
- Lysis of the enveloped viruses either by activation of classical pathway (most viruses) or some time by alternative or lectin pathways (by some viruses like Epstein–Barr virus, rubella virus, etc.).
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