0805 - Complement - PW

Question 1

What are the three activation pathways of the complement system?

Answer 1

Classical, Alternative and Lectin

Question 2

What is the initial stimulus for each?

Answer 2

Classical responds to antibodies IgG and IgM bound to their target. The lectin pathway responds to carbohydrates on bacteria and other microbes. The alternative pathway is activated by microbial surfaces

Question 3

Name the enzyme that is common to each of the three pathways.

Answer 3

C3 convertase

Question 4

Following complement activation, there is three possible routes by which the system can deal with pathogens. Describe these three.

Answer 4

1. recruitment of inflammatory cells - mast cell degranulation and neutrophil recruitment. 2. Opsonisation of pathogens - bacteria or cells labelled by complement proteins are tagged for phagocytosis 3. Direct killing of pathogens via the membrane attack complex

Question 5

Complement factors have two significant effects on B-cells. Describe these.

Answer 5

Complement lowers the threshold of B cell activation in response to a pathogen and complement proteins act in the germinal centre to enable affinity maturation.

Question 6

How are complement protein levels utilised in a clinical way to determine if a complement response is mounted and which pathway is activated?

Answer 6

If the complement system is activated, C3 and/or C4 will be consumed (i.e. the serum levels will fall). C3 will be lower whether the classical or alternative pathway is activated. C4 is low in addition to C3 if the classical pathway is activated.