Complement Pathway

The complement pathway refers to a series of tightly regulated, enzyme-driven cascades within the complement system—a crucial component of innate immunity. The system consists of more than 30 soluble and membrane-bound proteins, primarily synthesized by the liver but also produced by immune and epithelial cells. These proteins circulate in an inactive form in blood plasma and tissue fluids until activated by specific triggers such as pathogens or antibody-antigen complexes. There are three main pathways for complement activation: Classical, Lectin and Alternative. All three pathways converge at the formation of a central enzymatic complex called the C3 convertase, which cleaves C3 into active fragments—C3a (anaphylatoxin) and C3b (opsonin). This convergence amplifies immune responses against pathogens. Complement activity is tightly controlled by regulatory proteins present in plasma and on cell membranes. Dysregulation or deficiencies within any part of these pathways can result in increased susceptibility to infections, autoimmune diseases, inflammatory disorders, or tissue damage due to excessive inflammation.