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The Complement Cascade

The Complement Cascade. The Alternative Pathway. C3a. C3b. C3 breaks down to C3a and C3b spontaneously at a slow rate. Complement: The Alternative Pathway. C3. Next. C3b.B. FB. Forming C3b.B. Complement: The Alternative Pathway. C3b.

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The Complement Cascade

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  1. The Complement Cascade The Alternative Pathway

  2. C3a C3b C3 breaks down to C3a and C3b spontaneously at a slow rate Complement: The Alternative Pathway C3 Next

  3. C3b.B FB Forming C3b.B Complement: The Alternative Pathway C3b (In the presence of Mg2+ ions) Factor B binds to C3b Next

  4. C3b.Bb FD FB Forming C3b.Bb Complement: The Alternative Pathway C3b.B C3b Factor D cleaves C3b.B Next

  5. FP Microbial surface FB Complement: The Alternative Pathway C3b.Bb FD C3b.B C3b C3b.Bb is unstable, with a very short half life It is stabilised by FP which is active of microbial surfaces Next

  6. Complement: The Alternative Pathway • C3b.Bb is termed the Alternative C3 Convertase • C3b.Bb cleaves C3 to produce C3b • (C3a is released) • C3b is the first molecule of the Terminal Pathway Next

  7. FB Complement: The Alternative Pathway C3 C3b.Bb FD C3a C3b.B C3b C3b.Bb cleaves C3 producing C3a and C3b Next

  8. FB Complement: The Alternative Pathway C3 C3b.Bb • This is a positive feedback mechanism • It enables the amplification of the complement process • Positive feedback mechanisms are relatively rare because they are inherently unstable • Hence, positive feedback mechanisms have controls to regulate them and to maintain homeostasis. • Factor H performs this role in the Alternative Pathway FD C3a C3b.B C3b Next

  9. FB FH Complement: The Alternative Pathway C3 C3b.Bb FD C3a C3b.B In the absence of infection, FH binds C3b in preference to FB If a bacterial surface is present, FB binds C3b in preference to FH C3b C3b.H Factor H competes with FB to bind C3b forming C3b.H Next

  10. FB FH Complement: The Alternative Pathway C3 C3b.Bb FD FI C3a C3b.B C3b.i.H C3b C3b.H C3b.i Factor I binds C3b.H – forming C3b.i.H FH is then released – recycling FH Next

  11. FB C3c C3d FH Complement: The Alternative Pathway C3 C3b.Bb FD FI C3a C3b.B C3b.i.H C3b C3b.H C3b.i C3b.i. spontaneously breaks down to C3c and C3d Next

  12. FB Complement: The Alternative Pathway C3 C3b.Bb FD C3a C3b.B C3b C3b is the first component of the Terminal Pathway (Click Here)

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