Inherited deficiencies of the terminal components of human complement.

The particularly frequent occurrence of terminal complement deficiencies in patients with Neisserial infections suggests that the cytolytic activity of the complement system is important in resistance to Neisseria meningitidis. There are, however, geographical differences in the prevalence of terminal complement deficiency in patients with meningococcal disease. The data available suggest that either recurrent infection or infection with uncommon serogroups should alert the clinician in Western countries whereas recurrent disease is the important indicator in high risk endemic or epidemic areas. An association of terminal complement deficiencies with susceptibility to autoimmune diseases or non-Neisserial infections is doubtful. For a better understanding of complement deficiencies in relation to disease more accurate characterization of the defects involved will be helpful. Sensitive ELISA techniques and molecular biological assays will be needed. Thus it has been established that two types of deficiencies exist (at least for C6, C7 and C8): one with low but detectable amounts of the component and the other with a complete absence of the protein in question. The subtotal variety appears to show less association with Neisserial infection. Low amounts of functional terminal complement activity may be sufficient for many of its biological functions, suggesting that there is a wide "safety margin".