Complement

Question 1

What is the complement system?

Answer 1

A system of soluble and membrane-bound plasma proteins that act as innate immune effector molecules to detect and destroy microorganisms.

Question 2

Which arm of immunity does complement belong to?

Answer 2

Innate immunity (but it links innate and adaptive immunity)

Question 3

Why are complement proteins synthesised in an inactive form?

Answer 3

To prevent uncontrolled activation and damage to host tissues

Question 4

What are the 3 main outcomes of complement activation?

Answer 4

Opsonisation, inflammation, and lysis

Question 5

Name the 3 complement activation pathways?

Answer 5

1. CLASSICAL
2. LECTIN
3. ALTERNATIVE

Question 6

What triggers the classical pathway?

Answer 6

Antibody- antigen complexes (IgG or IgM)

Question 7

What triggers the lectin pathway?

Answer 7

Binding of MBL or ficolins to microbial carbohydrate structures

Question 8

What triggers the alternative pathway?

Answer 8

Spontaneous C3 activation amplified on pathogen surfaces

Question 9

What key component do all 3 pathways converge on?

Answer 9

C3

Question 10

What are the initiator molecules in the classical pathway?

Answer 10

C1q (with C1r and C1s)

Question 11

What are the initiator molecules in the lectin pathway?

Answer 11

Mannose-binding lectin (MBL) and ficolins

Question 12

What role does C1q play?

Answer 12

It recognises and binds antibody- antigen complexes

Question 13

What are C1r and C1s?

Answer 13

Serine proteases that cleave C4 and C2

Question 14

What are MASPs?

Answer 14

MBL- associated serine proteases that activate complement in the lectin pathway

Question 15

What is a convertase?

Answer 15

An enzyme complex that cleaves complement proteins to drive the cascade

Question 16

What is the C3 convertase of the classical and lectin pathways?

Answer 16

C4b2a

Question 17

What is the C3 convertase of the alternative pathway?

Answer 17

C3bBb

Question 18

What does C3 convertase do?

Answer 18

Cleaves C3 into C3a and C3b

Question 19

How is the C5 convertase formed in the classical pathway?

Answer 19

C4b2a + C3b = C4b2a3b

Question 20

How is the C5 convertase formed in the alternative pathway?

Answer 20

C3bBb + C3b = C3bBb3b

Question 21

What is opsonisation?

Answer 21

The coating of pathogens with molecules that enhance phagocytosis

Question 22

What is the main complement opsonin?

Answer 22

C3b

Question 23

How does C3b enhance phagocytosis?

Answer 23

It binds to complement receptors on phagocytes, improving pathogen recognition and uptake

Question 24

What are anaphylatoxins?

Answer 24

Small component fragments that induce inflammation

Question 25

Name the 3 complement anaphylatoxins?

Answer 25

C3a, C4a, C5a

Question 26

Which anaphylatoxin is the most potent?

Answer 26

C5a

Question 27

List 3 effects of anaphylatoxins?

Answer 27

1. Mast cell degranulation 2. Histamine release 3. Increased vascular permeability

Question 28

How do anaphylatoxins help immune cells reach infected tissue?

Answer 28

By increasing blood flow, vascular permeability and leukocyte extravasation

Question 29

Which complement component is the most powerful chemoattractant?

Answer 29

C5a

Question 30

How do C3b and C5a act together during phagocytosis?

Answer 30

C3b opsonises pathogens while C5a recruits and activates phagocytes

Question 31

What is the MAC?

Answer 31

A pore-forming complex composed of C5b-C9 that lyses target cells

Question 32

Which complement component initiates MAC formation?

Answer 32

C5b

Question 33

Why are multiple C9 molecules required?

Answer 33

To polymerise and form a stable transmembrane pore

Question 34

Against which pathogens is MAC most effective?

Answer 34

Gram-negative bacteria

Question 35

Why must complement be regulated?

Answer 35

To prevent host tissue damage and excessive inflammation

Question 36

What does C1 inhibitor (C1-INH) do?

Answer 36

Prevents activation of C1r and C1s (and MASPs)

Question 37

What is the role of CD55 (DAF)?

Answer 37

Dissociates C3 and C5 convertases

Question 38

What is the role of Factor H and Factor I?

Answer 38

Inactivates C3b on host cell surfaces

Question 39

What is CD59 (protectin)?

Answer 39

A regulatory protein that prevents MAC formation by blocking C9 polymerisation

Question 40

How do pathogens evade complement?

Answer 40

By inhibiting activation, binding host regulatory proteins, or preventing MAC formation

Question 41

Why does binding host complement regulators benefit pathogens?

Answer 41

It makes them appear as "self" and avoids immune attack

Question 42

Explain why complement is considered a bridge between innate and adaptive immunity?

Answer 42

Because the classical pathway is activated by antibodies, and complement enhances antibody-mediated phagocytosis and B-cell responses