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Flashcards in 4. Complement Deck (59):

what are the 5 important immune functions of complement?

1. opsonization (C3b and iC3b)
2. Cell Lysis (MAC)
3. Inflammation (C3a, C5a)
4. limiting immune complex-mediated destruction (complement receptors)
5. alert the adaptive immune system to pathogen invasion (C3d and C5a)


where are complement proteins (inactive) produced?

liver (and some blood cells, aka monocytes)


complement is activated by _____, which cleaves each C' protein into a and b components that do what?


one acts as a protease itself, activating the next step of the cascade
one will be inactive, or perform other immune functions


who are the susbtrate-binding molecules of complement?

C1q (bdins to antigen:Ab complexes and pathogen surfaces)

C3, C4, MBL (MBL binds to carb structures like mannose)


who are the serine proteases in complement?

C1r, C2s, C2a, Factor B (Bb), Factor D, MASP


what proteins make up the MAC?

C5b, C6, C7, C8, C9


who are the bioactive fragments in complement?

C3a, C5a, C3b, iC3b, (C4a)


who are the regulators in complement?

properidin (only positive regulator mentioned)

C1 inhibitor (C1INH)

Factor I (the only PROTEASE mentioned)

Factor H

C4-binding protein (C4BP)

membrane cofactor protein (MCP = CD46)

decay accelerating factor (DAF = CD55)

S protein



who are the complement receptors?

CR1 (CD35)
CR2 (CD21)
CR3 (CD11b)
CR4 (CD11c)


who are the homologues of C3?

C4, and C5 (and alpha 2 macroglubulin)


who are the homologues of Factor B (Bb)?

C2 (C2a)


who are the homologues of C1r?

C1s and MASP1&2


who are the homologues of CR3?

CR4 (Beta2 integrins)


who are the homologeus of Factor H?

CR1 (CD35) and C4BP


who are the homologes of C3aR?



what makes C3 (and C4) molecules unique?

they ar ethe only defense molecules known to bind covalently to the surface of pathogens via a THIOESTER BOND


which pathways generate the significant biologically active molecs (C3a, C3b, C5a, and MAC)?

all of them


The thioester bond in intact C3 is...

concealed - exposed in C3b following cleavage of C3a chain by C3 convertase


what happens to C3b in the fluid phase?

inactivated by hydrolysis


what are the serine proteases in unique to the classical pathway?

C1r, C1s, C2a


what are the unique substrate-binding molecules in the classical pathway?

C1q and C4


what is required to initiate the classical pathway cascade?

IgM or IgG ( IgG1 or IgG3) antibodies


what comprises the C1 complex?

6 C1q molecules and a tetramer of covalently associated C1r and C1s (two each: 2 C1r&s)


C1q binds to ______ of Ig with C1q's globular head regions, leading to what?

Fc region

leading to the activation (via conformational change) of C1r enzyme that then cleaves and activates C1s enzyme


what does activated C1s do?

cleaves soluble C4 into C4a and C4b, exposing a thioester site on C4b to allow it to covalently bind in the vicinity of the Ab/C1 complex and exposing another site on C4b, permitting non covalent association wht the potential enzyme C2.

C1s then cleaves C2 into C2b and the next active protease, C2a


what is the C3 convertase?

the C4a, C2b complex


what does C3 convertase do?

cleaves C3 into C3a and C3b

C3b then associates with the C4b, C2a dimer and a C4b, C2a,C3b tripartite complex forms, changing the protease specificity of C2a so that it becomes C5 convertase


what does C5 convertase do?

cleaves C5 into C5a and C5b


what does C5b/C5a production lead to?

C5b: initiation of the lytic cascade

C5a: promotes inflammation


which complement proteins are the anaphylatoxins?

C5a and C3a (inflammatory mediators)


what are the collectins?

MBL and ficolins (they are secreted by PRRs) (associate with MASP1/2)


the lectin pathway feeds into the C4/C2 classical pathway, but with the unique ___________ and two serine proteases called _______.

unique substrate-binding molecule mannose-binding lectin (MBL) (or ficolin) (PRRs)

serine proteases called MASP1 and MASP2


what happens when MBL binds mannose?

MASP1 is activated, which then activates MASP2


what does MASP2 cleave?

C4 and C2


what are the dedicated serine proteases to the alternate pathway?

Factor D and Bb (fragment of Factor B, a close homologue of C2)


what is C3 tickover?

small amount of C3 continually cleaved in the blood into C3a and C3b (usu no problem because the dangerous thioester site uncovered by the cleavage can be rapidly hydrolyzed by water, or thioester attachment to atoms on autologous cell membranes contain regulatory proteins that block the cascade in its tracks)


what happens if if C3b from tickover binds an atom (at NH2 or OH residues) at the cell surface?

will bind Factor B (must be stabilized by properdin/factor P)


what does Factor B do?

substrate for Factor D, which cleaves it into Ba and Bb


what is the active C3 convertase of the alternate pathway?

C3b,Bb (amplifies response and cleaves many C3 molecs into C3a and C3b)


what is the amplification step of the alternate pathway?

C3b,Bb cleaves many C3 into C3a and C3b


what is the C5 convertase of the alternate pathway?



C5a is a potent anaphylatoxin that does what?

induces smooth muscle contraction
increases vascular permeability
activate mediator-release by mast cells
initiate adaptive immune response

(highlighted functions from ppt above, all functions from notes below)

(induces vasodilation, chemoattraction, mast cell degranulation, induction of molecules on endothelia allowing attachment of leukocytes to vessel walls, induction of pro-inflammatory cytokines and even respiratory burst (C3a does the same thing))


what are 3 general functions of the regulators of the C' Cascade?

1. block responses to self tissues
2. prevent C' from being used up
3. limit responses to keep inflammation to a minimum


on host cells, which complement regulatory proteins bind to C3b?

CR1, H, MCP, and DAF (so that bound C3b is then cleaved by Factor I to yield iC3b)


on host cells which complement-regulatory proteins displace Bb?

CR1, H, and DAF


what does C1INH do?

binds to activated C1r, C1s, removing them from C1q and to activated MASP2, removing it from MBL


what does C4BP do?

binds C4b, displacing C2a; cofactor for C4b cleavage by I


what does CD59 do?

prevents formation of the MAC on autologous or allogenic cells. Widely expressed on membranes.


CR1 is found on which cells?

hematopoietic cells


how does CR1 function on phagocytes?

it is an opsonic receptor that bind to C3b when it is covalently associated with antigen/pathogen (poor at inducing phagocytosis on its own) and synergizes with Fc receptors, other complement receptors, or signals generated from C5a bidning to GPCRs (or cytokines like IFN-gamma) to enhance uptake and destruction of C2b or iC3b-coated cells


whta does the CR1 on erythrocytes do?

soaks up immune complexes (IC) bound covelently to C2b or iC3b and removes them from circulation (important because in the absence of such removal, the IC lodge in tissues causing damage that leads to hypersensitivity and autoimmunity)



till capable of opsonization through CR3 and CR4 but is not able to form a new convertase, i.e. it cannot recruit Factor B


which complement receptor has been recently shown to be suprioer to CR1 in its opsonic ability?



CR2 (CD21) - what does it do?

found on B cells and follicular dendritic cells and has the highest affinity for C3d (the small fragment of C3 that remains covalently attached to cell surfaces after Factor I completes its proteolytic treatement of C3), Binding of C3d lowers the activation potential of B cells so that suboptimal stimulation through the Ig receptor can lead to B cell stimulation - ie the innate system alerts the adaptive system to the presence of an infection


CR3 (CD11b/CD18) and CR4 (CD11c/CD18) ?

are both integrins, found predominantly on neutrophils and macrophages that bind with greatest affinity to iC3b. CR3 is a very important opsonic receptor, as C3b is usually ‘nicked’ down proteolytically to iC3b on the surface of pathogens.


how does Candida avoid C'?

coats itself with facotr H to block complement fixation on the surface


how does coating its surface with sialic acid help a bug evade C'?

surface doesn't promote the binding of Factor P, resulting itn eh stabilization of the C3Bf complex


how does Leishmania evade C'?

uses complement to invade cells - iC3b bound to surface of Leishmania provides an entry into macrophages after binding to CR3


how does measles virus evade C'?

uses MCP to enter susceptible cells