[Human resistance to generalized bacterial infections (exemplified by meningococcal infection)]

The hierarchical organization of human host defense systems against systemic bacterial infections is considered by using meningococcal disease as a model. The bactericidal action of the complement system is the most potent defense mechanism against meningococci. The antibody-independent alternative pathway of complement activation is more important in infancy. When the specific antibody level increases by natural immunization or vaccination, the antibody-dependent classical pathway of complement activation provides an additional protection. The bactericidal effect of human phagocytes, primarily neutrophils, is mediated partly by the receptors of complement components and immunoglobulins and serves as an additional mechanism of resistance. The relative risk of meningococcal disease may be approximately estimated as 1,000 for the individuals without blood complement bacteriolytic activity, as 80 for those without specific bactericidal antibodies, and as 3 for individuals with ineffective phagocytosis as compared to those with the normal complement system, high levels of bactericidal antibodies, and effective phagocytosis by neutrophils.