from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A protein in blood serum that participates in the body's immune response by working in conjunction with the alternative pathway of the complement system.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A protein in human serum that complements part of the immune system
The activation through the classical pathway requires C1q, the alternative pathway requires C3b, properdin, and serum factors B and D; and the lectin pathway requires binding carbohyrdate residues on the surface of the bacteria similar to C1q.