(kom'plĕ-mĕnt )

[L. complere, to complete]

A group of proteins in the blood that play a vital role in the body's immune defenses through a cascade of interactions. Components of complement are labeled C1 through C9. Complement acts by directly lysing (killing) organisms; by opsonizing an antigen, thus stimulating phagocytosis; and by stimulating inflammation and the B-cell–mediated immune response. All complement proteins lie inactive in the blood until activated by either the classic or the alternative pathways.

The lack of any of the more than 25 proteins involved in the complement system may affect the body's defenses adversely. Lack of C3, for example, increases susceptibility to common bacterial infections, and deficiencies in C5, C6, C7, and C8 increase susceptibility to invasive meningococcal disease, including meningococcal meningitis. Deficits of C5, C6, C7, C8, and C9 (the terminal components of complement) are often associated with autoimmune diseases, such as systemic lupus erythematosis and glomerulonephritis. Lack of C1 causes hereditary angioedema of the extremities and the gastrointestinal tract.