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The complement system 

The complement system
The complement system

Marina Botto

, and Matthew C. Pickering

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date: 02 March 2021

The complement system, consisting of soluble and membrane-bound proteins, is a major effector mechanism of host defence against infection and inflammatory responses. It has an important role in the removal of immune complexes and dying cells, and also modulates humoral and cell-mediated immune responses. Complement activation occurs through three pathways, each generating enzyme complexes, termed C3 convertases. These cleave native C3 to form C3b and C3a. C3b can covalently attach to surfaces (e.g. pathogen surfaces) where it triggers biological responses following interaction with membrane-bound receptors and can also trigger cleavage of native C5 to C5b and C5a. C5b triggers the formation of the membrane attack complex, which disrupts target cell membrane integrity and may result in cell lysis.

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