COMPLEMENT SYSTEM

Complement System

The complement system refers to a series of >20 proteins, circulating in the blood and tissue fluids. Most of the proteins are normally inactive, but in response to the recognition of molecular components of microorganisms they become sequentially activated in an enzyme cascade – the activation of one protein cleaves and activates the next protein in the cascade enzymatically. Complement can be activated through three different pathways,  each of which can cause the activation of C3, cleaving it into a large fragment, C3b, that acts as an opsonin, and a small fragment C3a (anaphylatoxin) that promotes inflammation. Activated C3 can trigger the lytic pathway, which can damage the plasma membranes of cells and some bacteria. C5a, produced by this process, attracts macrophages and neutrophils and also activates mast cells. Complement was discovered as a heat-labile component of normal plasma that causes the opsonisation and killing of bacteria.

Classical Pathway

This pathway involves complement components C1, C2 and C4. The pathway is triggered by antibody-antigen complexes binding to C1, which itself has three subcomponents C1q, C1r and C1s. The pathway forms a C3 convertase, C4b2a, which splits C3 into two fragments; the large fragment, C3b, can covalently attach to the surface of microbial pathogens and opsonise them; the small fragment, C3a, activates mast cells, causing the release of vasoactive mediators such as histamine.

Alternative Pathway

This pathway involves various factors, B, D, H & I, which interact with each other, and with C3b, to form a C3 convertase, C3bBb, that can activate more C3, hence the pathway is sometimes called ‘the amplification loop’. Activation of the loop is promoted in the presence of bacterial and fungal cell walls, but is inhibited by molecules on the surface of normal mammalian cells.

Mannose-binding Lectin Pathway

This pathway is activated by the binding of mannose-binding lectin (MBL) to mannose residues on the pathogen surface. This in turn activates the MBL-associated serine proteases, MASP-1 and MASP-2, which activate C4 and C2, to form the C3 convertase, C4b2a.

Role of Complement in Disease

The complement system plays a critical role in inflammation and defence against some bacterial infections. Complement may also be activated during reactions against incompatible blood transfusions, and during the damaging immune responses that accompany autoimmune disease. Deficiencies of individual complement components or inhibitors of the system can lead to a variety of diseases (Tabulated), which gives some indication of their role in protection against disease.

Table . Diseases associated with complement deficiencies

Complement Deficiency	Disease
C3 and Factor B	Severe bacterial infections
C3b-INA, C6 and C8	Severe Neisseria infections
Deficiencies of early C components C1, C4, C2.	Systemic lupus erythematosus (SLE)

REFERENCES:
Zaahira Gani, Cambridge, UK
https://www.immunology.org/public-information/bitesized-immunology/systems-and-processes/complement-system
https://www.youtube.com/watch?v=mfCeCvkQbuI&t=63s