Classical Pathway Overview

classical, lectin, alternative

the three pathways of the complement system

zymogen

name for inactive form of complements

C1q

what binds antibodies in the classical pathway

CH2

the domain of antigen-bound antibodies where C1q can bind

C1r₂s₂

complexes with C1q to form C1 complex

C1s

cleaves C4 so that C4b can bind C2

C4b

binds C2 to bring it to bring it to C1s

C1s

cleaves C2 into C2a (larger piece) forming C4b2a (C3 convertase)

C4b2a

C3 convertase full component name

C3 convertase

cleaves C3 into C3a and C3b, forming the C4b2a3b (C5 convertase)

C4b2a3b

C5 convertase full component name

C5 convertase

can cleave C5 forming C5a and C5b

CD21

C3b receptor when it is functioning as an opsonin

C3b

can also bind phagocytes or RBCs with antibodies for destruction

90

percentage of C4 that does not bind C2

thioester

bond that forms with cell membrane preventing C4b from binding C2

hydrolyzed

the form of C4 that cannot bind cell membrane

j chain

contributes to bent shape of IgM pentamer and IgG hexamer to expose complement protein binding sites