The complement system consists of about 30 proteins present in blood. These proteins cooperate with each other to attack foreign cells that have invaded the body. Complement proteins alert cells of the immune system that an infection has occurred. They signal the immune system to mount an attack. Individuals who lack any one of the complement proteins suffer from severe and recurrent infections.
Complement proteins bind to microorganisms either directly or indirectly.
Binding of complement initiates a cascade of reactions that lead to the following effects.