module 08 section 02 (classical pathway)

Question 1

recall: what is the classical complement pathway?

Answer 1

one of the three pathways that activates the complement system

Question 2

recall: what is the classical pathway activated by?

Answer 2

antigen-antibody complexes with IgG and IgM

Question 3

what is step 1 of the classical pathway?

Answer 3

• antibodies bind to a multivalent antigen on the cell membrane of the target cell
• the C1 complex (C1qr2s2) binds to the Fc portion of the antibody bound to the target cell

Question 4

explain the structure of the C1 complex

Answer 4

flower shape - similar in structure to collectin (in the lectin pathway)

Question 5

what is step 2 of the classical pathway?

Answer 5

• inactive circulating C4 then binds to the C1q portion of the Ig-associated C1 complex
• r2s2 enzyme then cleaves C4 into C4a and C4b

Question 6

what is step 3 of the classical pathway?

Answer 6

• C4b covalently attaches to cell membrane or antibody
• C2 binds to C4b
• C1qr2s2 cleaves C2 into C2a and C2b

Question 7

what is step 4 of the classical pathway?

Answer 7

-C4b and C2a combine to form C3 convertase (C4bC2a)

Question 8

why is C3 convertase called C3 convertase?

Answer 8

because its the complex that converts C3 protein

Question 9

what is step 5 of the classical pathway?

Answer 9

C3 convertase binds to and cleaves C3 into C3a and C3b

Question 10

what is step 6 of the classical pathway?

Answer 10

• C3b moecules bind to C4bC2a to form C5 convertase (C4bC2aC3b)
• unbound C3a can either be hydrolyzed (where it becomes inactive) or form covalent bonds with target cell surfaces

Question 11

what is step 7 of the classical pathway?

Answer 11

• C5 convertase binds to C5 and cleases it into C5b

- recall that C5b is the end protein that all three complement pathways create