A Comprehensive Review of the Complement System: Molecular Mechanisms, Regulatory Networks, and Therapeutic Applications

The complement system represents a sophisticated immune surveillance network that bridges innate and adaptive immunity through a cascade of proteolytic reactions. Comprising over 50 proteins, this system provides first-line defense against pathogens, clears immune complexes, and maintains tissue homeostasis. However, dysregulated complement activation contributes significantly to the pathogenesis of autoimmune disorders, thrombotic microangiopathies, and inflammatory diseases. This review offers a comprehensive examination of the complement system's molecular architecture, focusing on the intricate mechanisms of its three activation pathways and their convergence on common effector functions. We detail the critical regulatory networks that maintain complement homeostasis and prevent host tissue damage. The review systematically analyzes the pathological consequences of complement dysregulation across various disease states and discusses the revolutionary advances in complement-targeted therapeutics. Finally, we explore emerging research directions and the future landscape of complement-based diagnostics and treatments, providing insights into the evolving understanding of this complex biological system.