Complement Pathways and Meningococcal Disease: Diagnostic Aspects

Complement is an immunological effector system that bridges innate and acquired immunity in several ways. There is a striking association between susceptibility to meningococcal disease and various forms of complement deficiency (1 ,2 ). In defense against bacterial infection, the most important function of complement is probably to serve as a mediator of antibody-dependent immunity. Specific antibodies can trigger activation of the classical and the alternative pathways of complement activation (3 –5 ). It is well known that antibody-independent mechanisms interfere with alternative pathway activation on the bacterial surface (6 ,7 ). The newly discovered mannan-binding lectin (MBL) pathway of complement activation appears to be protective against many types of infection (8 ) and adds previously unsuspected aspects of innate immunity to complement-mediated defense. Interestingly, immune responses are influenced by complement (9 ), and it could be that acquisition of protective antibodies is impaired in some types of complement deficiency. A further aspect of interactions between Neisseria and complement is the potential role of membrane-bound complement regulators as cellular receptors for the microbes (7 ).