| Complement component C5 is a central effector protein of the complement cascade that links upstream C3 convertase activity to generation of potent anaphylatoxin and cytolytic responses, and belongs to the family of large modular complement proteins that circulate mainly as inactive precursors until they are cleaved by C5 convertases on activating surfaces. The molecule consists of two chains derived from a single precursor and organized into multiple β‑sheet‑rich domains homologous to those in C3 and other terminal complement components, creating surfaces that support recognition by C5 convertases and that, after cleavage, yield a soluble effector fragment and a membrane‑binding fragment that initiates terminal complex assembly. Conversion of C3 into C3b during classical, lectin, or alternative pathway activation allows assembly of pathway‑specific C5 convertases, which bind native C5 and cleave it into the small fragment C5a and the larger fragment C5b; C5a diffuses away and acts as a strong anaphylatoxin, whereas C5b remains associated with the activation site and nucleates formation of the membrane attack complex. C5a engages specific G protein–coupled receptors such as C5aR1 on neutrophils, monocytes, macrophages, and other immune and non‑immune cells, where it triggers chemotaxis, degranulation, upregulation of adhesion molecules, oxidative burst, and release of inflammatory mediators, linking complement activation to cell recruitment, vascular permeability changes, and amplification of local and systemic inflammatory responses. C5b sequentially recruits C6, C7, C8, and multiple C9 molecules to assemble the C5b‑9 membrane attack complex that inserts into susceptible membranes and forms transmembrane pores, leading to osmotic imbalance and lytic death of bacteria and other complement‑sensitive targets, and contributing to clearance of damaged or altered host cells under certain conditions. Regulation of C5 activation and its effector functions is mediated by fluid‑phase and membrane‑bound inhibitors such as factor H–controlled upstream convertase regulation, vitronectin and clusterin binding to soluble C5b‑9, and CD59 on host cells that blocks C9 polymerization, collectively restricting terminal complex assembly and protecting host tissues from complement‑mediated damage while preserving antimicrobial efficacy. Functional roles of C5 and its fragments extend into modulation of adaptive immunity, as C5a–C5aR signaling influences dendritic‑cell maturation, T helper cell polarization, and B cell responses, integrating complement activation with shaping of antigen‑specific immunity and tolerance. Dysregulated C5 activation or impaired control of C5b‑9 assembly contributes to pathology in disorders such as paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, membranous nephropathies, age‑related macular degeneration, and a range of autoimmune and inflammatory diseases, where excessive terminal pathway activity drives hemolysis, endothelial injury, and tissue inflammation. |
