Changes

[[Image:Complement Classical Pathway.png|thumb|right|250px|'''Classical pathway in detail''' - R.J.Francis, RVC 2012]]

[[Image:Complement Classical Pathway.png|thumb|right|250px|'''Classical pathway in detail''' - R.J.Francis, RVC 2012]]

This pathway of the complement system is triggered by the binding of antibody to antigen. The only subclasses that can activate complement this way are the

This pathway of the complement system is triggered by the binding of antibody to antigen. The only subclasses that can activate complement this way are the

[[IgM]] and certain [[IgG]]. The first step of complement activation is the binding of C1 to two Fc regions of the antibodies (IgM is such a strong activator of complement as it contains '''five''' Fc regions, while IgG contains '''one'''). C1 is actually a complex of C1q, C1r and C1s. C1q looks like a bunch of 6 tulips with each "flower" consisting of a globular protein head and a collagen "stem". <font color="green">C1q is required to trigger the cascade because two of it is the globular heads binds the two Fc regions.</font color> C1r and C1s then become activated when the C1q heads are antibody-bound. When they are activated they form the enzyme '''C1 esterase''' (it is a serine protease).

[[IgM]] and certain [[IgG]]. The first step of complement activation is the binding of C1 to two Fc regions of the antibodies (IgM is such a strong activator of complement as it contains '''five''' Fc regions, while IgG contains '''one'''). C1 is actually a complex of C1q, C1r and C1s. C1q looks like a bunch of 6 tulips with each "flower" consisting of a globular protein head and a collagen "stem". C1q is required to trigger the cascade because two of its globular heads bind to the two Fc regions. C1r and C1s then become activated when the C1q heads are antibody-bound. When they are activated they form the enzyme '''C1 esterase''' (it is a serine protease).

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==Functions==

==Functions==

===Opsonisation===

===Opsonisation===

Once activated the complement system deposits a shell of protein on the bacterial cell surface. C4b, C3b, C5b and C7 molecules contain active binding sites that anchor the complex to the surface and the major protein on the cell surface is iC3b (some of the smaller C3 breakdown products (e.g. C3d) are also present).

Once activated the complement system deposits a shell of protein on the bacterial cell surface. C4b, C3b, C5b and C7 molecules contain active binding sites that anchor the complex to the surface and the major protein on the cell surface is iC3b (also known as C3bi) (some of the smaller C3 breakdown products (e.g. C3d) are also present).

Complement fragments (C2b, C3a, C4a and especially C5a) released after complement activation are chemotactic for phagocytes and iC3b acts as a target for [[Phagocytosis|phagocytosis]]. Phagocytes have receptors that bind avidly with the complement fragments. Complement-mediated opsonisation of microorganisms is several thousand times more efficient that innate receptors.

Complement fragments (C2b, C3a, C4a and especially C5a) released after complement activation are chemotactic for phagocytes and iC3b acts as a target for [[Phagocytosis|phagocytosis]]. Phagocytes have receptors that bind avidly with the complement fragments. Complement-mediated opsonisation of microorganisms is several thousand times more efficient than innate receptors.

===Inflammation===

===Inflammation===

<p>The smaller complement peptides are very efficient at inducing inflammation i.e. C3a and C5a. They attract [[Blood Cells - Overview|granulocytes]] to the site of complement activation as well as stimulating their degranulation.</p>

The smaller complement peptides are very efficient at inducing inflammation i.e. C3a and C5a. They attract [[Blood Cells - Overview|granulocytes]] to the site of complement activation as well as stimulating their degranulation.</p>

===Cell Lysis===

===Cell Lysis===

{{Learning

{{Learning

|flashcards = [[Complement Flashcards]]

|flashcards = [[Complement Flashcards]]

}}

}}

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