Complement

Question 1

Classical pathway

Answer 1

Triggered by fixation of C1 to antibody labeled antigen (IgM or IgG on bug)

Question 2

Alternative pathway

Answer 2

Triggered by microbial surface molecules. LPS, endotoxin, complex polysacharides, venom, etc. in the absence of antibody.

Question 3

Lectin pathway

Answer 3

Mannose-binding lectin (in plasma) binds to carbohydrates on microbes and directly activates C1

Question 4

C3 convertase

Answer 4

• All 3 complement activation pathways lead to formation of active C3 convertase
• C3 convertase splits C3 into fragments C3a and C3b
• C3a released
• C3b binds to previous fragments to form c5 convertase

Question 5

Functions of the complement system

Answer 5

1. Inflammation
2. Opsonization and phagocytosis

Cell lysis

Question 6

C5 convertase

Answer 6

• Formed from C3b + other fragments.
• Cleaves C5 to release C5a and leave C5b attached to the cell surface
• C5b binds late components (C6-9) and ultimately forming the MAC

Question 7

MAC

Answer 7

Membrane attack complex

Composed of miultiple C9 molecules

C3 –> C3b –> C5 convertase

C5 –> C5b –> binds with C6-C9

Multiple C9 ——> MAC

Question 8

Anaphylatoxins

Answer 8

C3a, C5a, C4a

complement components that stimulate histamine release from mast cells (^^^ vascular permeability, vasodilation)