Complement Deficiency

Complement is a group of serum proteins that helps the body fight infection. Activated complement coats bacteria so phagocytes can clear them, draws inflammatory cells to the site and, through the terminal components C5 to C9, assembles the membrane attack complex that punches holes in the outer membrane of Neisseria bacteria. Complement deficiency means that one or more of these proteins is missing or works poorly. It is a defect of one arm of innate immunity: T cells and antibody production usually work normally, so the condition is very different from combined immunodeficiency such as SCID. Primary (inherited) complement deficiency can involve a single component (for example C2, C3 or one of C5 to C9), a regulator (for example factor H, factor I or C1 inhibitor) or properdin. The ESID/ERN RITA complement guideline notes that most heritable complement deficiencies are autosomal recessive, and it recommends testing siblings and other potentially affected relatives, so relatives with similar infections or symptoms warrant attention. Properdin deficiency is the only X-linked complement deficiency, so it mainly affects males. Acquired complement deficiency is more common...