Complement

Question 1

What roles does the complement system play?

Answer 1

Recognises pathogen/ danger signals + triggers responses that can directly lyse pathogens, opsonise pathogens, act as a chemoattractant, trigger degranulation of mast cells

Question 2

How can complement proteins be measured?

Answer 2

In serum of all animals

Question 3

Describe the different modes of action of complement proteins

Answer 3

• direct inhibitors/ blockade
• decay accelerator factors- e.g. CD55
• Found on many WBCs surfaces speeds up the decay of convertases
• CD59 inhibits the formation of MAC on host surfaces

Question 4

Name the 3 complement pathways

Answer 4

• Classical pathway
• Alternative pathway
• MB-lectin pathway

Question 5

How do the complement pathways differ?

Answer 5

Classical = reliant on Ab being produced, longer
Alternative = quickest, binds directly to pathogen surface
MB-lectin = reliant on another protein for activation (mannose), binds surface via soluble protein

Question 6

What is the activating signal of classical complement pathway?

Answer 6

Ag:Ab complex

Question 7

What is the activating signal of the alternative complement pathway?

Answer 7

C3 binding to pathogen carbohydrates/ proteins

Question 8

What is the activating signal of the MB-lectin complement pathway?

Answer 8

Host mannose- binding lectins: pathogen mannose

Question 9

What are the effectors of the complement pathways?

Answer 9

cell phagocytosis, cell lysis, WBC recruitment + activation

Question 10

What are targets of the complement pathways?

Answer 10

bacteria, viruses, parasites, fungi

Question 11

How is C3 made?

Answer 11

Made by macrophages in the liver

Question 12

What is the highest concentration of serum complement proteins?

Answer 12

C3

Question 13

Describe the breakdown of C3?

Answer 13

C3 breaks down naturally into C3a + C3b

• C3a acts as an anaphylatoxin/ chemoattractant
• C3b can bind microbe surfaces via CHO, C3b can bind Factor H on host cells taking C3b out of circulation

Question 14

Which complement proteins are anaphylatoxins?

Answer 14

C3a, C4a, C5a

Question 15

What effect do anaphylatoxins have?

Answer 15

smooth muscle contraction, increases vasodilation, activates mast cells/ neutrophils, increases fluid in the tissue + speeds up lymph flow

Question 16

What is the Membrane Attack Complex

Answer 16

Central effector mechanism of complement system + immune response

Question 17

Explain the Membrane Attack Complex

Answer 17

= endpoint of all 3 activation pathways

- composed of C5b, C6, C7, C8 + multiple copies of C9

Question 18

What does the MAC do?

Answer 18

opens bacterial cell cytosol = loss of cellular homeostasis, disrupts protein gradient (signalling), penetration of host lysozyme + other proteases