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lisapoor

Complement reaction

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Question... can anyone help me in describing the general complement reaction in terms of a cascade. Also what is the end result of complement activation? and what are some of the other functions of complement components?

 

I appreciate any help. :) Lp

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The complement system is a complex cleavage cascade involving many C proteins (C1-9), if you need un-necessary detail a good immunology text will cover this. Generally you should only know the importance of the activation of C3 convertase to cleave C3 to create C3b! (via classical or alternate pathways). C3b is a major product that then permits further cascades, resulting in enhanced phagocytosis of antigen (e.g organism) known as opsonisation, or lysis of target via membrane attack complex formation, and initiation of acute immflamatory reactions. The latter is important for enhanced immune responses and C-reactive protein is vital for responses to bacterial infection (activation of complement).

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i love the complement system. it will you take some time to get your head around all its subtleties but it is well worth the effort. the complement system is one of many examples which reveal how extracellular signalling involve cascades of protease reactions.

 

as DrZoidberg writes, the formation of C3b is essential for activating the complement cascade. C3b is a protein fragment of C3, but it is very unstable and undergoes self-proteolysis to another fragment called iC3b or C3d which is inactive, thus limiting the extent of immune responses. there are also special cell surface proteins, eg CD46 and CD59, whose function is to increase the rate of C3b decay.

 

interestingly mycobacteria coat themselves in iC3b so that they are taken up by host cells.

 

interestingly the proteins C3b and C5b have domains homologous to alpha-2-macroglobulin, the archetypal extracellular protein dustbin.

 

i could go on for hours - hope this helps.

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