Hi!

We are a study group on our second year of medical laboratory science, and our immunology exam is coming up. There is some confusion though.

 

When C1q binds the antibody, C1r is activated and then cleaved and activates C1s. Then C1s will cleave C4 and C2. C4b and C2a will then bind to the pathogen and the antibody, and then C4b2a is formed. Now, the question is:

 

Is C3b2a a C3-convertase or a C3/C5-convertase? Our textbook says C3/C5-convertase, but I've been studying a bit on the Internet as well, and there I've read that it's a C3-convertase. What is correct? Doesn't C3b2a turn into C3/C5-convertase when it binds another C3b and becomes C3b2a3b-complex? Our textbook says nothing about C3b binding to C3b2a, just that C3b2a is C3/C5-convertase.

 

I hope this makes sense, thanks for any help!

Are you talking about C4b2a or C3b2a? C4b2a and C3bBb are both C3 convertases - C3 convertase is the point at which both the classic and alternative cascades converge. Once C4b2a cleaves C3, C4b2a3b is the C5 convertase, while C3a is a pyrogen.