Final answer:
The complement system is a group of proteins synthesized by the liver and macrophages, abundant in the blood serum, and plays a crucial role in both the innate and adaptive immune responses against pathogens.
Step-by-step explanation:
The complement system consists of approximately 20 soluble proteins that are primarily responsible for helping to destroy extracellular pathogens, such as bacteria and viruses. These complement proteins are synthesized continuously by liver cells and macrophages, making them abundantly present in the blood serum where they can rapidly respond to infectious agents.
The system is an integral part of both the innate and adaptive immune responses, with a mechanism that involves a highly regulated, sequential binding and activation of complement proteins, leading to a cascade effect whereby pathogens become coated and marked for destruction. This process enhances the ability of phagocytes to engulf pathogens, a phenomenon known as opsonization, and can also result in the formation of membrane attack complexes that lyse the pathogens.