Complement

Complement is a series of >20 plasma and membrane proteins.•It an evolutionarily old system for pathogen recognition.•It has three main pathways of activation:

Classical pathway:Activated predominantly by Ig-containing complexes.Alternative pathway:Spontaneously activated on ‘permissive surfaces’.Lectin pathway:Activated by lectin-carbohydrate interactions.

•All three pathways converge at the activation of the central component ‘C3’

The functions of complement include:

• making bacteria more susceptible to phagocytosis

• directly lysing some bacteria and foreign cells

• producing chemotactic substances

• increasing vascular permeability

• causing smooth muscle contraction promoting mast cell degranulation

• One of the functions of complement is the deposition of C3 on the surface of

• particles.• •C3-opsonised material is phagocytosed by C3

receptors.• The major receptor being complement receptor 3 (CD11b/CD18).• Internalisation by CR3 does not trigger microbicidal responses

• Complement receptor mediated phagocytosis