The complement system plays a crucial role in innate immunity and is now recognized as a bridge between innate and adaptive immune responses. It is a complex network of plasma and membrane-associated proteins that elicit efficient and tightly regulated inflammatory and cytolytic immune responses. Complement activation leads to opsonization, lysis of pathogens, and the production of proinflammatory molecules, which are essential for host defense. Recent studies have shown that complement also interacts with T- and B-cell biology, influencing adaptive immune responses. The complement system can be activated through three pathways: classical, lectin, and alternative. The classical pathway is initiated by the binding of C1q to antibody attached to pathogens, leading to the formation of C3 convertase. The lectin pathway recognizes conserved pathogenic carbohydrate motifs, while the alternative pathway is activated by spontaneous hydrolysis of C3. Complement regulation is essential to prevent excessive activation and collateral damage to healthy tissues. Inhibitory proteins such as Factor I, DAF, and CR1 help control complement activation. The complement system has three major effector pathways: direct lysis of pathogens via the membrane attack complex (MAC), generation of anaphylatoxins that induce inflammation, and opsonization of pathogens for phagocytosis. Complement also plays a critical role in adaptive immunity, particularly in B-cell regulation and humoral immunity. Complement receptors such as CR1 and CR2 are essential for B-cell signaling and antibody production. Complement also influences T-cell immunity by modulating T-cell activation and responses. Pathogens have evolved strategies to evade complement, such as producing proteins that inhibit complement activation or interfere with complement effector functions. Complement deficiency can lead to increased susceptibility to infections, highlighting its importance in host defense. Complement is an essential component of innate immunity and plays a vital role in the immune response to pathogens.The complement system plays a crucial role in innate immunity and is now recognized as a bridge between innate and adaptive immune responses. It is a complex network of plasma and membrane-associated proteins that elicit efficient and tightly regulated inflammatory and cytolytic immune responses. Complement activation leads to opsonization, lysis of pathogens, and the production of proinflammatory molecules, which are essential for host defense. Recent studies have shown that complement also interacts with T- and B-cell biology, influencing adaptive immune responses. The complement system can be activated through three pathways: classical, lectin, and alternative. The classical pathway is initiated by the binding of C1q to antibody attached to pathogens, leading to the formation of C3 convertase. The lectin pathway recognizes conserved pathogenic carbohydrate motifs, while the alternative pathway is activated by spontaneous hydrolysis of C3. Complement regulation is essential to prevent excessive activation and collateral damage to healthy tissues. Inhibitory proteins such as Factor I, DAF, and CR1 help control complement activation. The complement system has three major effector pathways: direct lysis of pathogens via the membrane attack complex (MAC), generation of anaphylatoxins that induce inflammation, and opsonization of pathogens for phagocytosis. Complement also plays a critical role in adaptive immunity, particularly in B-cell regulation and humoral immunity. Complement receptors such as CR1 and CR2 are essential for B-cell signaling and antibody production. Complement also influences T-cell immunity by modulating T-cell activation and responses. Pathogens have evolved strategies to evade complement, such as producing proteins that inhibit complement activation or interfere with complement effector functions. Complement deficiency can lead to increased susceptibility to infections, highlighting its importance in host defense. Complement is an essential component of innate immunity and plays a vital role in the immune response to pathogens.