9. Complement Pathways

Question 1

What is complement? How is it synthesized?

Answer 1

Complement = a system of serum proteins that interact with Ag-Ab complexes or components of microorganisms to eliminate Ags through cell lysis and other bioogical mechanisms

Synthesized = by macrophages, monocytes, intestinal epithelial cells, and hepatocytes

Question 2

What classes of antibody are able to activate the classical pathway of complement?

Answer 2

• IgG (IgG3 > IgG1 > IgG2 ; NOT IgG4)

- IgM (Better at binding complement than IgG)

Question 3

What complement proteins comprise the recognition unit in the classical pathway?

Answer 3

C1 = C1q , C1r, C1s

Question 4

What complement proteins are involved in formation of the activation unit in the classical pathway ?

Answer 4

• Activation unit = C4, C2, C3
• Forms C4b,2a,3b
• C1s starts by cleaving C4 and C2

Question 5

What complement proteins comprise the membrane attack complex?

Answer 5

C5b, C6, C7, C8, C9

Question 6

How is the lectin pathway activated, and how is it different from the classical pathway?

Answer 6

• activated by certain microorganisms which have the sugar Mannose on their cell surface
• does not require antibodies for activation

Question 7

How is the alternative pathway of complement activated?

Answer 7

• Zymosan (yeast cell wall polysaccharide)
• Lipopolysaccharide (LPS; gram negative bacteria)
• Teichoic acid (gram positive bacteria)
• Others

Question 8

What complement proteins are involved in both the classical and alternative pathways?

Answer 8

C3, C5, C6, C7, C8, C9

Question 9

What complement proteins are unique to the alternative pathway?

Answer 9

• Factor B
• Factor D
• Properdin (P)

Question 10

What complement proteins make up C3 convertase in the classical pathway? in the alternative pathway?

Answer 10

Classical Pathway = C4b,2a

Alternative Pathway = C3bBb