Neisseria meningitidis (Gram-negative cocci) life threatening cause of sepsis and bacterial meningitis.
Classically, individuals who lack terminal complement proteins (C5-C9) cannot form the membrane attack complex (MAC), which is needed for bacterial lysis of this encapsulated pathogen (can lead to a 1000- to 10,000-fold increased risk of meningococcal disease). More information here.
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Rosa D.D., Pasqualotto A.C., de Quadros M., Prezzi S.H. Deficiency of the eighth component of complement associated with recurrent meningococcal meningitis—case report and literature review. Braz J Infect Dis. 2004;8(August (4)):328–330.