Final answer:
The complement system refers to a series of proteins found in the blood plasma that enhance the immune response. They are synthesized in the liver and play a vital role in both the innate and adaptive immunity by opsonizing pathogens and aiding in their destruction.
Step-by-step explanation:
The complement system is composed of a combination of proteins found in blood plasma, which "complement" certain immune responses. These proteins are not considered part of the early induced immune response but share features with some of the antibacterial proteins of this class. The proteins within the complement system are synthesized mainly in the liver.
The system is involved in both the innate and adaptive immune responses using pathways referred to as the "alternate pathway" and the "classical pathway." Complement proteins, once activated, partake in a sequence of enzymatic reactions known as a cascade, leading to irreversible actions. These actions include the opsonization of pathogens which labels them for phagocytosis, or 'eating' by other immune cells.
The complement system is critical due to its abundance in blood serum and the capability to respond immediately to infecting microorganisms. It functions by binding to the surfaces of microorganisms, especially those already bound by antibodies, and is crucial for enhancing phagocytosis and aiding the adaptive immune response when antibodies are produced.