Components of complement

Component	(mcg/mL)	Function
Classical pathway (CP)
C1	50	Binds immune complexes to trigger classical pathway
C1q subcomponent		Binds Fc portion of IgG/IgM
C1r subcomponent		Protease: Cleaves C1s
C1s subcomponent		Protease: Cleaves C4 and C2
C4	200 to 500	Opsonin* (C4b), C4a^¶
C2	20	Protease: Cleaves C3 and C5 as part of the convertases
Lectin pathway (LP)
MBL^Δ	150 (wide range)	Binds sugars to trigger LP
MASP-1	5	Protease: Cleaves MASP-2 and C4
MASP-2	5	Protease: Cleaves C4 and C2
Alternative pathway (AP)
Factor D	1 to 2	Protease: Cleaves factor B to Ba and Bb
Factor B	150 to 250	Protease: Cleaves C3 and C5 as part of the convertases
Properdin	25	Stabilizes AP convertases
Central protein
C3	550 to 1200	Opsonin* (C3b), C3a^¶
Terminal pathway (TP)
C5	70	MAC component is C5b; C5a^¶(anaphylatoxin) is released
C6	60	MAC component
C7	60	MAC component
C8	60	MAC component (pore formation)
C9	60	MAC component (pore formation)

IgG: immunoglobulin G; IgM: immunoglobulin M; MBL: mannose-binding lectin; MASP: MBL-associated serine protease; MAC: membrane attack complex.
* C4b forms part of the classical pathway C3 and C5 convertases. It anchors the enzyme complex to the target. Likewise, for the alternative pathway C3 and C5 convertase, C3b anchors these enzyme complexes to the target. C3b is also part of the classical pathway and lectin pathway C5 convertase.
¶ C4a, C3a, and C5a anaphylatoxins are liberated upon cleavage of C4, C3, and C5.
Δ MBL is the most commonly measured collectin (normal value shown).

Graphic 68272 Version 9.0

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Components of complement