Classical complement pathway: Difference between revisions

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The newly formed C4b cannot stay activated as a highly reactive thioester bond is revealed once C4 has been cleaved. The thioester bond is cleaved by water resulting in its cleavage permanently deactivating the C4b molecule. As a result of this C4b is restricted to only bind to pathogen surfaces. They would undergo rapid deactivation in the time it took to travel from the origin of activation where C1q is complexed with an antigen-antibody immune complex(IC) or where C1q is directly attached to the pathogens surface.<ref name=":1">{{Cite book |last=Janeway |first=Ca Jr |url=https://www.ncbi.nlm.nih.gov/books/NBK27100/ |title=Immunobiology: The Immune System in Health and Disease |chapter=The complement system and innate immunity |publisher=Garland Science |year=2001 |edition=5th |___location=New York}}</ref> the pathogen.

=== Formation of CC3-3 convertase. ===

Surface bound C4b acts as a receptor for the binding of C2.<ref name=":1" /> The binding of C2 and C4b results in C2 being cleaved by C1s into C2a and C2b. C2a diffuses into the plasma as a protein inflammatory mediator while C2b remains attached with C4b forming C4bC2b what is known as C3-convertase, The function of the membrane-bound C3-convertase is the cleavage of many many molecules of C3 into C3a and C3b. C3a is a smaller fragment of C3 is a potent inflammatory mediator.

=== C3b function and structure. ===

C3b can act as an opsonin . C3b is very similar to C4 in both structure and function also has a thioester bond that forces it to attach to surface nucleophile of the activator(namely the pathogen or IC). Phagocytes have receptors for C3b and as a result of receptor-ligand binding are able to more easily recognize and engulf pathogen molecules. While the [[anaphylatoxin]] C3a interacts with its [[C3a receptor]] (C3aR) to recruit leukocytes, C3b contributes to further downstream complement activation.<ref name="Overview of Complement" /><ref name="Complement history" />

===Formation of C5 convertase and MAC===