Complement component 3, often simply called C3, is a member of the complement system. The complement system is an important mediator of natural and acquired immunity. It consists of approximately 30 proteins that can exhibit catalytic activity, function as regulators, or act as cellular surface receptors. These components normally circulate in inactive forms and are activated by the classical, alternative, or lectin pathways. Complement component 3 plays a central role in all 3 activation pathways. People with C3 deficiency are susceptible to bacterial infection. Clinically, the Level of C3 in the blood may be measured to support or refute a particular medical diagnosis. For example, low C3 levels are associated with some types of kidney disease such as post-infectious glomerulonephritis and shunt nephritis.