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Phase 2a - Introductory Clinical Sciences > Immunology > Flashcards

Flashcards in Immunology Deck (160)
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what causes initial cleavage of C3 in the alternative pathway?

occurs spontaneously, generating low levels of C3b


once formed, what does C3b do in the alternative pathway?

uses factors D and B to produce active enzyme C3bBb, which is stabilised by properdin


what is the action of C3bBb in alternative pathway? what stabilises it?

breaks down more C3, providing more C3b.


what regulates the breakdown of C3 in the alternative pathway?

factor H and I
H competes with factor B for binding to C3b, then I cleaves and inactivates displaced C3b


what initiates the lectin pathway?

mannose-binding lectin - circulating protein, binds to carbohydrate of surface of some microorganisms


what are the two ways in which C5 convertase is produced?

in classical = C3b, C4b and C2b
in alternative = C3b, Bb and properdin


describe the final lytic pathway of complement?

once C5 convertase is produced, C5 is cleaved into C5a and C5b.
successive addition of C6/C7/C8/C9 forms the MEMBRANE ATTACK COMPLEX.
- lysis of cell via pore formation.


how does complement-dependent phagocytosis work?

microorganisms are coated with C3b - can then be bound by possessing complement receptors (CR1) - present on phagocytic cells


what pro-inflammatory mediators does complement activation release? what are their effects?

C5a, C4a and C3a.
act as anaphylatoxins.
increased vascular permeability, release vasoactive amines, induce smooth muscle spasm.


what is the action of C5a?

acts as anaphylatoxin.
also, a potent chemoattractant, and it stimulates neutrophils and macrophages to synthesise cytokines, undergo oxidative metabolism and release degradative enzymes.