The complement system plays a crucial role in both innate and adaptive immune responses. Initially thought to be solely involved in innate immunity, recent studies have revealed its significant impact on adaptive immune responses. Complement activation leads to robust proteolytic cascades that terminate in opsonization and lysis of pathogens, as well as the generation of proinflammatory molecules. The system is composed of over 30 plasma and membrane-associated proteins, organized into classical, lectin, and alternative pathways. These pathways converge on the formation of convertases, which generate anaphylatoxins, the membrane attack complex (MAC), and opsonins. Complement regulation ensures that these processes are confined to appropriate pathogenic surfaces, minimizing bystander tissue damage. Complement's effector functions include direct lysis, inflammation, and opsonization, which are essential for host defense against common pathogens. In adaptive immunity, complement influences B-cell differentiation and function, enhancing B-cell signaling and antibody production. It also modulates T-cell responses, potentially through anaphylatoxin receptors. Pathogens have evolved strategies to evade complement, such as producing inhibitors or altering their surface properties. Understanding the roles of complement in host immunity is crucial for developing therapeutic strategies against microbial infections, autoimmune disorders, and organ transplantation.The complement system plays a crucial role in both innate and adaptive immune responses. Initially thought to be solely involved in innate immunity, recent studies have revealed its significant impact on adaptive immune responses. Complement activation leads to robust proteolytic cascades that terminate in opsonization and lysis of pathogens, as well as the generation of proinflammatory molecules. The system is composed of over 30 plasma and membrane-associated proteins, organized into classical, lectin, and alternative pathways. These pathways converge on the formation of convertases, which generate anaphylatoxins, the membrane attack complex (MAC), and opsonins. Complement regulation ensures that these processes are confined to appropriate pathogenic surfaces, minimizing bystander tissue damage. Complement's effector functions include direct lysis, inflammation, and opsonization, which are essential for host defense against common pathogens. In adaptive immunity, complement influences B-cell differentiation and function, enhancing B-cell signaling and antibody production. It also modulates T-cell responses, potentially through anaphylatoxin receptors. Pathogens have evolved strategies to evade complement, such as producing inhibitors or altering their surface properties. Understanding the roles of complement in host immunity is crucial for developing therapeutic strategies against microbial infections, autoimmune disorders, and organ transplantation.