"Complement proteins are identified by the letter 'C' and a number, the most common complement tests being C3, C4, and CH50. The classic pathway complement function markers of your immune system are Complement C3 and C4.
Complement is a blood protein that destroys bacteria and helps control inflammation. Your doctor is looking for evidence of a chronic, inflammatory disorder of the immune system. "
Hope this explains it a little bit, but the link Inga gave is great too.