Complement sytem

Question 1

What are the distinct phases of the complement system which result in the elimination of microbes?

Answer 1

• pattern recognition trigger
• protease cascade amplification/C3 convertase
• end processes

Question 2

What are the 3 complement pathways?

Answer 2

• classical pathway
• MB-lectin pathway
• alternative pathway

Question 3

What is the trigger for the classical pathway?

Answer 3

antigen-antibody complexes

Question 4

What are the components of the classical pathway?

Answer 4

What are the components of the classical pathway?

Question 5

What is the trigger for the MB-lectin pathway?

Answer 5

Mannose-binding lectin binds mannose on pathogen surfaces

Question 6

What are the components of the MB-lectin pathway?

Answer 6

MBL, MASP-1, MASP-2
C4
C2

Question 7

What are the components of the alternative pathway?

Answer 7

Pathogen surfaces

Question 8

Describe the classical pathway of the complement system

Answer 8

1. C1q binds to IgG antibody on antigen surface = ‘complement fixing’
2. Activates C1r which cleaves C1s
3. Activated C1s binds to C4 & enzymatically liberates C4a & C4b - C4b binds to antigen surface
4. This also causes cleavage of C2 into C2a & C2b
5. C2a binds to C4b
6. Forms C3 convertase on antigen surface
7. This splits C3 into C3a & C3b
8. C3b joins C4bC2a complex to form C5 convertase

Question 9

What are some consequences of complement activation?

Answer 9

1. Inflammation
2. Removal of particulate antigens & immune complexes (enhances phagocytosis)
3. Cytolysis - formation of Membrane Attack Complex (MAC)

Question 10

What effects can C3a & C5a have on cells/in the body, in terms of causing inflammation?

Answer 10

Smooth muscle contraction
Mast cell degranulation
Vasodilation > local oedema, local influx of antibody & complement, cell extravasation
Neutrophil activation

Question 11

How does activation of the complement system cause removal of particulate antigens & immune complexes?

Answer 11

1. Phagocytosis by non-specific adherence (macrophage)
2. Opsonisation (phagocytic cells express receptors for IgG & C3b)
3. Removal of immune complexes (in spleen/liver)

Question 12

What is the function of the Membrane Attack Complex (MAC)?

Answer 12

Allows influx of water into cells > leads to osmotic lysis