Complement C5 deficiencies in humans are rare, but often associated with recurrent infections by Neisseria spp. (such as meningitis). Deficiency in C5 results in an impairment of hemolytic activity from both the classical and alternative pathway of the complement system. Recently, genetic research has revealed that a 153 bp deletion in exon 30 results in complete C5 deficiency. Research has also implicated C5 deficiency in a predisposition to cardiac dysfunction when cardiac injury occurs.
Complement C5 deficiency can, paradoxically, protect against certain effects of infection. Patients deficient in C5 are unusually susceptible to meningococcal infection, but have a milder course of infection. Reduced expression of tumor necrosis factor (TNF) may account for this effect.
