C h a p t e r 1 5
Innate and Adaptive Immunity
329
2
3
Early-Step
Inflammatory
Responses.
The central compo-
nent of complement for all three
pathways is the activation of the
complement protein C3 and its
enzymatic cleavage into larger C3b
fragments and smaller C3a frag-
ments. The smaller 3a fragments
stimulate inflammation by acting as
a chemoattractant for neutrophils.
The larger 3b fragment becomes
attached to the microbe and acts as
an opsonin for phagocytosis. They
also act as an enzyme to cleave C5
(see next step).
Late-Step Membrane Attack.
In the late-step responses, C3b
binds to other complement proteins
to form an enzyme that cleaves
C5, generating C5a and C5b frag-
ments. C5a stimulates the influx of
neutrophils and the vascular phase
of acute inflammation. The C5b
fragment, which remains attached
to the microbe, initiates the forma-
tion of a complex of complement
proteins C6, C7, C8, and C9 into a
membrane attack complex protein,
or pore, which allows fluids and
ions to enter and cause cell lysis.
C3
C5
C3a
C3b
C3b
C3b
C5a
3b bound
to microbe
Chemotaxis
Vascular dilation
and permeability
Phagocytosis
C5
C5b
C5b
C6,C7,C8,C9
Lysis of
microbe
Membrane
attack
complex
