Complement system

The complement system is a network of blood and tissue proteins that amplifies immune responses, marks microbes for destruction, and can rupture susceptible cells.

Classical pathway: A complement activation route initiated when antibodies or certain proteins bind a target. Antibodies link adaptive recognition to complement activation through this route.
Complement component 3: A central complement protein cleaved into the inflammatory mediator C3a and the opsonin C3b. Its cleavage connects pathway activation to both inflammation and pathogen tagging.
Neisseria meningitidis: A bacterium that can cause meningitis and bloodstream infection, especially when terminal complement defense is impaired. Deficiency in terminal complement components greatly increases susceptibility to this bacterium.
Antibody: An immune protein that binds specific antigens and can recruit other immune mechanisms. Antibodies can initiate the classical pathway, but complement also activates without them.
Lectin pathway: A complement activation route triggered when lectins bind characteristic carbohydrates on microbial surfaces. It recognizes microbial sugars without requiring antibodies.
C3b: A complement fragment that coats targets, promotes phagocytosis, and helps form convertases. Its surface deposition marks microbes and drives further complement activation.
Paroxysmal nocturnal hemoglobinuria: An acquired stem-cell disorder in which blood cells lack protective surface proteins and undergo complement-mediated destruction. Complement blockade reduces the red-cell destruction that defines this disease.
Phagocytosis: The cellular process of engulfing and digesting particles, microbes, or debris. Complement often marks pathogens for phagocytosis rather than engulfing them itself.
Alternative pathway: A complement activation route in which spontaneous C3 activation is amplified on suitable surfaces. This route amplifies complement deposition directly on pathogen surfaces.
C5a: A potent inflammatory complement fragment that attracts and activates immune cells. It recruits leukocytes to sites where complement is activated.

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