The complement system, a part of the immune system, consists of a series of plasma proteins, primarily including C1 through C9, Factor B, Factor D, and others. These components are mainly synthesized in the liver and circulate in the blood and bodily fluids. The complement system is activated through the classical pathway, alternative pathway, and lectin pathway, participating in biological processes such as immune defense, inflammatory responses, and cell lysis. Activation of the complement system promotes pathogen clearance, enhances phagocytosis, and regulates immune cell function. The gene expression of complement components is specific, and mutations or functional defects are associated with various immune deficiencies, autoimmune diseases, and inflammatory conditions. Therefore, the complement system is not only a critical component of innate immunity but also an important area for studying disease mechanisms and developing therapeutic targets.