Changes

===Alternative Pathway===

===Alternative Pathway===

[[Image:Complement Alternative Pathway.png|thumb|right|250px|'''Alternative pathway in detail''']]

[[Image:Complement Alternative Pathway.png|thumb|right|250px|'''Alternative pathway in detail''']]

<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> This pathway is found in all animals, including those that do not have antibodies i.e. animals lower in order than cartilaginous fish. It was discovered after the classical pathway and was thus called the 'alternative pathway'.

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

 

**The surfaces of certain activators stabilise the otherwise short-lived C3b in the absence of antibody

***Gram-negative bacteria, yeasts and fungi are the most efficient activators

*When active C3b is bound to particle surfaces, it is protected from inactivation by another complement component, properdin

To begin the cascade C3b binds to a surface that stabilises the C3b. C3b can bind to any surface as it has a binding site that anchors it to any surface, however it is only the surfaces of certain activators that can stabilise the otherwise short-lived C3b in the absence of antibody. These include Gram-negative bacteria, yeasts and fungi. When the active C3b is bound to particle surfaces, it is protected from inactivation by another complement component, properdin. Therefore, having the C3b bound to a surface enables the C3b to become active for longer. The C3b can then bind Factor B producing the complex C3bB. While in this complex, Factor B can become a substrate for the plasma enzyme known as Factor D. Factor D then splits a small peptide (the Ba peptide) from Factor B to generate the '''C3 convertase (althernative pathway)''' made of C3b&macr;Bb.  

*C3b can then bind Factor B producing the complex C3bB

*C3bB is the only substrate for a plasma enzyme known as Factor D.  

*Factor D splits a small peptide (the Ba peptide) from Factor B

This C3 convertase (C3b&macr;Bb) then acts on C3 to generate more C3b to then generate more C3b&macr;Bb: This process enables an efficient positive feedback loop that propagates the response. The C3b&macr;Bb can also bind C3b to form C3b&macr;Bb&macr;3b which itself is a C5 convertase and is one of the two enzymes that activates the [[#Membrane Attack Complex|membrane attack complex]] (MAC). This occurs in the same way as the clasical pathway, with the C5 convertase splitting C5 into C5b, that activates the complex, and C5a, the potent chemoattractant.

**This generates C3b&macr;Bb. (C3 convertase)

<br />

**C3b&macr;Bb acts on C3 to generate more C3b

<br />

**C3b then generates more C3b&macr;Bb

At the same time as C3b&macr;Bb&macr;3b is being formed, Factors I and H are acting to breakdown C3b to iC3b which, as mentioned above, forms an opsonin for [[Phagocytosis|phagocytosis]].

***An efficient positive feedback loop

 

*C3b&macr;Bb can also bind C3b to form C3bB&macr;b3b.

**C3bB&macr;b3b is one of the two enzymes that activates the [[#Membrane Attack Complex|membrane attack complex]] (MAC)

*C3bB&macr;b3b splits C5 into:

**C5b, the initiator of the MAC</p>

<p>

*At the same time as C3bB&macr;b3b is being formed, Factors I and H are acting to breakdown C3b to iC3b

***In plasma or on bacterial surfaces.

[[Image:Complement Mannose Binding Lectin Pathway.png|thumb|right|250px|'''Mannose Binding Lectin pathway in detail''']]

[[Image:Complement Mannose Binding Lectin Pathway.png|thumb|right|250px|'''Mannose Binding Lectin pathway in detail''']]

<p>The main effects of alternative complement activation are therefore:

<br />

The main effects of alternative complement activation are therefore:

#To coat bacteria with iC3b

#To coat bacteria with iC3b

#*A major target for [[Phagocytosis|phagocytosis]] by [[Macrophages|macrophages]] and [[Neutrophils|neutrophils]] via the complement receptors

#*A major target for [[Phagocytosis|phagocytosis]] by [[Macrophages|macrophages]] and [[Neutrophils|neutrophils]] via the complement receptors