Classical Pathway

Activation of C1
↓
Binding of C1 to Ab & Ag complex
↓
C1q binds to at least 2 Fc
↓
Conformational change (C1q)
↓
Activates C1r and C1s (form C1 complex, bound by Ca ions)
↓
Activated C1s cleave C4
↓
(2 arrows forming C2a) Forms C4a and C4b
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C1s and C4b cleaves C2
↓
C4b-C2b form complex (C3 convertase) [this is where classical, alternative and lectin pathway meet]
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*Convertase activate C3
↓
(2 arrows forming C3a) C3b
↓
C3b and C4b2b forms C5 convertase
↓
(2 arrows forming C5a) C5b
↓
C5b bind C6 and C7
↓
Gather C8, forms small pores
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Forms pores with C9 [formation of the Membrane Attack Complex (MAC)]

*One C3 convertase can actually cleave many C3, this is thus the amplification step