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Flashcards in Complement system Deck (28):

What does the activation of the complement pathway involved?

Sequential proteolytic cleavage of substrates that results in a cascade reaction


what are the 4 main functions of the complement system?

1. lyse target cells
2. promote phagocytosis 3. induce inflammatory response
4.clear immune complexes


activation of the classical complement pathway requires one or more:

antibody-antigen complexes


What is the alternative complement pathway involved in?

Innate immunity and can function in the absence of antibody


What triggers the alternative pathway ?

A range of microbial molecules as well as nonpathogenic molecules


What initiates the alternative pathway?

spontaneous hydrolysis of C3
-C3b then deposits on the microbial surface


Which protein is key in maintaining the activation of the alternative pathway by stabilization?

-stabilizes C3bBb
-also called factor P


What is the lectin pathway involved in ?

Innate immunity
-does not depend on the presence of antibody


What activates the lectin pathway?

Interaction of mannose binding lectin with mannose residues on glycoproteins/carbohydrates that are expressed by certain microorganisms


what are the 2 MBL associated proteases generally found on the surface of microbes?

MASP-1 and MASP-2


What is the role of MASP 1 and MASP 2

bind MBL on the microbe to form a complex that is able to cleave C4 and C2


Where can the complement system be regulated?

Before AND after the assembly of C3 convertase
-very important to regulate it so that only the appropriate targets are being attacked


What is C1 inhibitor and what does it do?

Induces dissociation and inhibition of C1r2s2 from C1q

is a serine protease inhibitor

affects the classical and lectin pathways


What is Decay Accelerating Factor (DAF)

Accelerates the dissociation of C4b2a and C3bBb (both the C3 convertases)


What is CR1 (CD55)

a membrane bound regulator of the complement pathway

Block formation of and accelerated dissociation of the C3 convertases by binding C4b and C3b


What is the role of Protectin ?

Binds C5b678 on host cells to prevent the binding of C9 and the formation of the MAC


What are 5 ways to turn off the complement pathway?

1. Dissociation of C1 components

2. Decay accelerating activity (DAF) for C3 convertases

3. Factor I cofactor activity (CR1)
-degrades C3b and C4b`

4. Inhibition of lysis/MAC (protectin)

5. Cleavage of anaphylatoxins by carboxypeptidases


What is the role of anaphylatoxins?

To attract leukocytes to the site of inflammation as well as to increase permeability in the vasculature


What are the 3 anaphylatoxins?

C3a, C4a, C5a


What are the special roles of C3a and C5a?

chemotactic for granulocytes and monocytes

Also cause platelet aggregation


Opsonization of microbes is achieved by?

C3b, C3bi, C4b


What is the role of RBC in the complement pathway?

CR1 on RBC interacts with C3b and carries small soluble complexes to the liver or spleen to be eliminated by phagocytes


CR1, 2, 3, and 4 are?

Receptors that bind complement components and their breakdown parts


What does CR1 bind and where is it expressed?

Binds: C3b, C4b, C1q, iC3b

Found on: RBC, neutrophils, monocytes, macrophages, eosinophils, FDCs, B cells, and some T cells


What does CR2 bind and where is it expressed?

aka Epstein-Barr virus receptor

Binds: C3d, C3dg (products of factor I cofactor activity)

Found on: B cells and FDCs


What does CR3 bind and where is it expressed?

Binds: iC3b and factor H

Found on: monocytes, macrophages, neutrophils, NK cells, eosinophils, FDCs, T cells


What does CR4 bind and where is it expressed?

Binds: iC3b

Found on: monocytes, macrophages, neutrophils, NK cells, T cells


What is the consequence of a breakdown int he clearance mechanism for complement?

some of the immune complexes deposit in the kidneys
-sometimes can occur post strep throat