L19/20 - Effector Mechanisms in Humoral Immunity I

Question 1

What are the effector mechanisms of antibodies?

Answer 1

• Opsonization
• Neutralization
• Activation of Complement System

Question 2

Where are complement components found and how are they activated?

Answer 2

Found as inactive plasma proteins widely distributed in tissues and body fluids and activated by proteolysis

Question 3

What does complement do?

Answer 3

• Promote lysis of bacteria, infected cells, transplanted cells
• Promote opsonization
• Promote inflammation

Question 4

How many / what produces complement proteins?

Answer 4

More than 30 proteins

- Produced by liver cells, monocytes and macrophages and some epithelial cells e.g. skin

Question 5

Which fragment is the smaller fragment, ‘a’ or ‘b’?

Answer 5

a is the smaller fragment except for C3, C3b is the smaller fragment

Question 6

What are the three pathways of complement activation?

Answer 6

• Classical
• Alternative
• Lectin

Question 7

What is the C3 convertase for the classical pathway?

Answer 7

C4bC2a

Question 8

What is the C3 convertase for the lectin pathway?

Answer 8

C4bC2a

Question 9

What is the C3 convertase for the alternative pathway?

Answer 9

C3bBb

Question 10

What is added to change a C3 convertase to a C5 convertase?

Answer 10

C3b. C4bC2aC3b and C3bBbC3b

Question 11

Explain the steps of the alternative pathway

Answer 11

1. Natural low level hydrolysis of C3 (tickover) leads to intermediates that cleave C3 to C3b and C3a
2. C3b can bind to microbial surfaces and binds to B
3. B is cleaved by Factor D to Bb (C3bBb convertase)
4. Properdin binds and stabailizies the complex

Question 12

What factor cleaves B?

Answer 12

Factor D

Question 13

What stabilizies the C3 convertase on microbial surfaces?

Answer 13

Properdin

Question 14

What degrades C3b in fluid phase?

Answer 14

Factor I and H

Question 15

Can C3b bind to host cell surfaces?

Answer 15

Yes, but is rapidly inactivated by complement regulatory proteins

Question 16

What makes up the C1 complex?

Answer 16

C1q, C1r and C1s

Question 17

What activates C1q and what is the result?

Answer 17

IgM or IgG antibodies or C reative protein (CRP). Induces C1r to cleave C1s to create an active protease

Question 18

Which IgGs are the best for complement activation?

Answer 18

IgG1 and IgG3

Question 19

What are the conditions of C1q binding to IgM and IgG?

Answer 19

• IgM must be antigen bound (comformational change)

- Must be bound to 2 IgGs

Question 20

Explain the steps of the classical pathway?

Answer 20

• C1q binds cell bound Ab and activates C1r2s2
• Activated C1s cleaves C4 (homologous to C3)
• C4b binds to the surface, binds C2 (cleaved by C1) and a C4bC2a convertase forms

Question 21

Explains the steps of the lectin pathway

Answer 21

• Involves MBL (similar to C1q), binds to MASP (C1rC1s) producing C4a and C4b
• C4b binds to C2, which is cleaved by MASP-2

Question 22

What is the result of C5 convertase?

Answer 22

Cleavage of C5 into C5a, C5b. Recruitment of C6, C7, C8 and C9 pore onto C5b forming MAC

Question 23

What are three functions of the complement system?

Answer 23

• Opsonization and phagocytosis
• Stimulation of inflammatory reactions
• Complement mediated cytolysis

Question 24

How can complement stimulate the inflammatory reaction?

Answer 24

Via C5a and C3a which are anaphylatoxin

Question 25

What are the opsonins of the complement system

Answer 25

C3b and C4b

Question 26

Which receptors have high affinity for C3b and what does this mean?

Answer 26

CR1 (CD35) and CR3, macrophages and neutrophils. High functions for phagocytosis

Question 27

How does CR1-mediated phagocytosis work?

Answer 27

Enhanced by specific IgG also binding to microbe. C3b/CR1 binding and IgG/FcγR binding

Question 28

How does CR1 help clear immune complexes with respect to erythrocytes?

Answer 28

Erythrocytes have CR1 complexes which bind C3b and antibody/C1q complexes which can be removed by the spleen and liver

Question 29

What is CR2?

Answer 29

Complement receptor 2 (CD21). A co-receptor for B-cell activation and also a receptor for Epstein Barr virus
- BCR binding to microbial antigen and CR2 to bound C3d enhances BCR signal

Question 30

How do anaphylatoxins cause inflammation?

Answer 30

Bind to C3a and C5a receptors found on mast cells, endothelial cells and phagocytes
- Induce TNFα release and histamine release by mast cells

Question 31

What additional role does C5a have?

Answer 31

It is a chemoattractant acting on neutrophils and monocytes

Question 32

What are some regulatory elements of the complement system?

Answer 32

• C1 inhibitory molecule C1 INH
• DAF, CR1 and MCP regulate C3 convertase production
• Factor I (and H) cleaving (inactivating C3b)
• CD59 (inhibits MAC formation)

Question 33

How does C1 INH inhibit C1 complex?

Answer 33

Prevents C1r2s2 from becoming proteolytically active, also inactivates MASP2

Question 34

How does inhibition of C3 convertase work?

Answer 34

DAF, MCP (in classical) and CR1 displace the 2 C3 convertase components. E.g. displacing C2b from C4b or Bb from C3b

Question 35

How does Factor I mediated cleavage work?

Answer 35

MCP and CR1 act as cofactors for Factor I, mediating proteolytic cleavage of C3b producing iC3b

Question 36

How does CD59 work?

Answer 36

Inhibits poly-C9 assembly

Question 37

How do microbes evade complement?

Answer 37

• Recruitment of host complement regulatory proteins. E.g. sialic acid, synthesise proteins to recruit factor H, incorporation of host regulatory proteins. e.g. DAF and CD59
• Inhibition of complement mediated inflammation

Question 38

What are some diseases of complement deficiency?

Answer 38

• Complement: deficiency in C1, C2 and C4 (systemic lupus erythematosus SLE)
• Alternative: B, D and properdin. Increased infection with Neisseria bacteria
• C3: Increased infection with Strep pneumoniae, Neisseria spp.