Porth's Essentials of Pathophysiology, 4e

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Innate and Adaptive Immunity

C h a p t e r 1 5

2

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).

C3

3b bound to microbe

C3b

C3a

C3b

C3b

Chemotaxis

C5

Phagocytosis

Vascular dilation and permeability

C5a

3

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.

C5

Membrane attack complex

C5b

C5b

C6,C7,C8,C9

Lysis of microbe