Changes

[[Image:LH Complement pathway.png|thumb|right|250px|'''Classical and alternative pathways''']]

[[Image:LH Complement pathway.png|thumb|right|250px|'''Classical and alternative pathways''']]

===Classical Pathway===

===Classical Pathway===

<p>Triggered by the binding of antibody to antigen. Only [[IgM]] and certain [[IgG]] subclasses can activate complement this way.

<p>Triggered by the binding of antibody to antigen. Only [[IgM]] and certain [[IgG]] subclasses can activate complement this way.

#Immune complexes trigger complement activation by binding C1.

#Immune complexes trigger complement activation by binding C1.

#*The binding of one C1q molecule produces one C1 esterase molecule that then causes the binding of many hundreds of C4b&macr;2b molecules

#*The binding of one C1q molecule produces one C1 esterase molecule that then causes the binding of many hundreds of C4b&macr;2b molecules

#The C3b molecule that is produced by the action of this enzyme can also bind to it

#The C3b molecule that is produced by the action of this enzyme can also bind to it

#*Forms C4b2&macr;b3b (C5 convertase)

#*Forms C4b2&macr;a3b (C5 convertase)

#*This activates C5 and initiates the [[#Membrane Attack Complex|membrane attack complex]] (MAC)

#*This activates C5 and initiates the [[#Membrane Attack Complex|membrane attack complex]] (MAC)

*Ca fragments C3a, C4a and C5a, and Cb fragment C2b

*Ca fragments C3a, C4a and C5a, and Cb fragment C2b

===Alternative Pathway===

===Alternative Pathway===

<p>Is found in animals that do not have antibodies i.e. animals lower than cartilaginous fish. It was discovered after the classical pathway and is thus called the 'alternative pathway'.

<p>Is found in animals that do not have antibodies i.e. animals lower than cartilaginous fish. It was discovered after the classical pathway and is thus called the 'alternative pathway'.

*C3b has a binding site that anchors it to any surface

*C3b has a binding site that anchors it to any surface

===MB-Lectin Pathway===

===MB-Lectin Pathway===

<p>Lectins are proteins that bind carbohydrates e.g. collectins. They bind carbohydrates that have a terminal mannose residue and are called mannose-binding lectins.

<p>Lectins are proteins that bind carbohydrates e.g. collectins. They bind carbohydrates that have a terminal mannose residue and are called mannose-binding lectins.

*Secreted by the liver into plasma.  

*Secreted by the liver into plasma.  

* The action of lectin binding to carbohydrate activates plasma-associated proteases called mannose-binding lectin associated proteases (MASPs)

* The action of lectin binding to carbohydrate activates plasma-associated proteases called mannose-binding lectin associated proteases (MASPs)

** These act on C4 and C2 in the same way as C1 esterase</p>

** These act on C4 and C2 in the same way as C1 esterase, and on C3 in much the same manner as C3 convertase</p>

==Membrane Attack Complex==

==Membrane Attack Complex==

[[Image:LH Complement lysis.png|thumb|200px|right|'''Membrane Attack Complex''']]

[[Image:LH Complement lysis.png|thumb|200px|right|'''Membrane Attack Complex''']]

This is the lytic pathway of complement function and can be initiated via either C3bB&macr;b3b or C4b2&macr;b3b.  

This is the lytic pathway of complement function and can be initiated via either C3bB&macr;b3b or C4b2&macr;a3b.  

#Split C5 into:

#Split C5 into:

#*C5a is pro-inflammatory.

#*C5a is pro-inflammatory.

===Opsonisation===

===Opsonisation===

<p>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).</p>

<p>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).</p>

<p>Complement fragments (C2a C3a, C4a and especially C5a) released after complement activation are chemotactic for phagocytes and iC3b acts as a target for 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.</p>

<p>Complement fragments (C2b, C3a, C4a and especially C5a) released after complement activation are chemotactic for phagocytes and iC3b acts as a target for 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.</p>

===Inflammation===

===Inflammation===