C5a

C5a is a protein fragment produced during the activation of the complement system, a crucial part of the immune system. The complement system consists of over 30 proteins that work together to defend the body against infections. These proteins circulate in the blood and fluids, usually inactive, until a signal triggers a chain reaction, activating them in sequence.

There are three main pathways for complement activation: the classical, lectin, and alternative pathways. All three converge at the activation of a key protein called C3. When C3 is activated, it splits into two fragments: C3a and C3b. C5a is a fragment derived from the activation of C5, which is downstream of C3. C5a, like C3a, plays a significant role in promoting inflammation by attracting immune cells to the site of infection or injury.12