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Scheme of the complement system. The complement system, also known as complement cascade, is a part of the humoral, innate immune system and enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane. [1]
The classical pathway is distinct from the other complement pathways in its unique activation triggers and cascade sequence. Activation of the complement pathway through the classical, lectin or alternative complement pathway is followed by a cascade of reactions eventually leading to the membrane attack complex.
Three biochemical pathways activate the complement system: the classical complement pathway, the alternate complement pathway, and the mannose-binding lectin pathway. [9] These processes differ only in the process of activating C3 convertase, [10] which is the initial step of complement activation, and the subsequent process are eventually the ...
Membrane attack complex (Terminal complement complex C5b-9) A membrane attack complex attached to a pathogenic cell The membrane attack complex (MAC) or terminal complement complex (TCC) is a complex of proteins typically formed on the surface of pathogen cell membranes as a result of the activation of the host's complement system, and as such is an effector of the immune system.
[8] Ligation of Fc receptors on the surfaces of immune effector cells can give rise to a number of responses, [9] such as degranulation (e.g., of mast cells, causing histamine liberation and subsequent urticaria), phagocytosis, release of pro-inflammatory cytokines and chemokines, platelet activation [10] resulting in the formation of clots, etc.
The alternative pathway is a type of cascade reaction of the complement system and is a component of the innate immune system, a natural defense against infections. The alternative pathway is one of three complement pathways that opsonize and kill pathogens. The pathway is triggered when the C3b protein directly binds a microbe. It can also be ...
It mediates inflammation by regulating leukocyte adhesion and migration and has been implicated in several immune processes such as phagocytosis, cell-mediated cytotoxicity, chemotaxis and cellular activation. [5] It is involved in the complement system due to its capacity to bind inactivated complement component 3b (iC3b). [8]
Complement-dependent cytotoxicity (CDC) is an effector function of IgG and IgM antibodies.When they are bound to surface antigen on target cell (e.g. bacterial or viral infected cell), the classical complement pathway is triggered by bonding protein C1q to these antibodies, resulting in formation of a membrane attack complex (MAC) and target cell lysis.