Complement system
1. Pathogen enters the body A bacterium or virus invades. The body recognizes it as “foreign” through immune surveillance. 2. Complement system activates There are three pathways: classical, lectin, and alternative . All converge to activate C3 , which splits into: C3a → small fragment, acts as signal . C3b → binds to pathogen surface (opsonization, marks it for phagocytosis). Later, C5 is cleaved into C5a and C5b : C5b → starts formation of MAC (Membrane Attack Complex) , which can punch holes in pathogen. C5a → one of the most powerful chemotactic signals . 3. Chemotactic signal is released C3a and C5a diffuse around the infection site . These fragments act like a “chemical smell” for immune cells. They bind to receptors on neutrophils, macrophages, and other phagocytes. 4. Immune cells move toward the signal Neutrophils detect higher concentration of C5a and move up t...