Complement

Question 1

Classical pathway

Answer 1

Antigen-antibody complexes

Question 2

Lectin

Answer 2

Lectin binding to pathogen surfaces

Question 3

Alternative

Answer 3

Pathogen surface. Always active

Question 4

Define opsonisation

Answer 4

Identifying the invading particle to phagocyte. Informs that it is dangerous and needs to be destroyed.

Question 5

Describe the structure of C1q

Answer 5

Has globular heads that binds actively to complexes.

Arms are very flexible and can move independently.

Activates 2 enzymes (C1r and C1s)

Question 6

Describe the classical pathway

Answer 6

C1q binds to the enzyme complex and activates C1r and C1s.

C1r cleaves C1s to activate protease.

The activated C1s cleaves the C4 molecule into C4a and C4b. C4b binds to microbial surface.

C1s then cleaves the C2 molecule into C2a and C2b. C2a binds to C4b to form C4b2a.

C4b2a is a C3 convertase, it cleaves C3 into C3a and C3b.

C3b binds to either the convertase or the bacterial surface.

Question 7

Name the 3 activation pathways of complement

Answer 7

Classical
Lectin
Alternative

Question 8

What is C4b2a?

Answer 8

An active C3 convertase

Question 9

What is C5 convertase made up of?

Answer 9

A C3b molecule joining to C4b2a

Question 10

Describe the mannose-binding lectin pathway.

Answer 10

MASP1 cleaves C4 into C4a and C4b.
C4b binds to the microbial surface.

MASP2 then cleaves C2 into C2a and C2b. C2a binds to the C4b to form C4b2a.

C4b2a is an active convertase. It cleaves C3 into C3a and C3b.

C3b can either bind to the microbial surface, or to the C3 convertase and become a C5 convertase.

C5 is cleaved to form C5a and C5b

Question 11

What is MASP?

Answer 11

Mannose-binding associated serine protease.

It is ridgid and will bind to mannose and fuccose residues that are in a particular pattern.

Question 12

Describe the alternative pathway.

Answer 12

C3 undergoes spontaneous hydrolysis into C3(H2O)

Factor B is cleaves by factor D into Ba and Bb.

Bb binds to C3(H2O) and is an active C3 convertase. Factor P stabilises this complex.

It cleaves more and more C3 molecules into C3a and C3b.

C3b will rapidly inactivate if not bound to cell surface.

On pathogen surface, another C3b will bind to the C3bBb complex to become a C5 convertase.

C5 is cleaves by the convertase complex into C5a and C5b.

Question 13

What is C5b?

Answer 13

Initiator for membrane attack.

Initiates signal pathway for the formation of pores on microbial surface.