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Background: Opsonophagocytosis and complement-mediated Neisseria meningitidis killing after vaccination were investigated., Methods: Twelve seronegative healthy volunteers received one dose of polysaccharide A/C vaccine and were followed for 3 years. Ex vivo serum killing rates with polymorphonuclear cells (PMN) and/or complement were performed at 0, 1.5, 6, 12, 18, 24, 30 and 36 months., Results: High mean total and median bactericidal antibodies were detected over time in all subjects. Considerable reduction of the initial inoculum was obtained only in the presence of complement, with or without PMN (with significant differences compared to curves without complement) a long time after vaccination., Conclusion: PMN did not increase post-vaccination bacterial killing, suggesting that antibody complement-mediated killing, and not opsonophagocytosis, is the main immune effector of the vaccine protection against N. meningitidis.

Background: To determine the anti-meningococcal C immunological activity by adding functional tests (opsonophagocytosis) to the classical serology techniques., Subjects and Methods: 42 adult volunteers were screened using serological methods (determination of total and bactericidal antibodies). Seronegative subjects were tested by opsonophagocytosis., Results: 24 subjects (57%) showed serological evidence of previous contact with Neisseria meningitidis group C antigens: 19 subjects had both total and bactericidal antibodies, two subjects had only total antibodies and three subjects had only bactericidal antibodies. Of the 18 seronegative subjects, five showed ex vivo activity in killing curves with or without polymorphonuclear cells: two subjects exhibited only complement-mediated bactericidal activity, one subject only opsonophagocytosis, and two subjects exhibited both activities., Conclusion: The addition of functional tests to the classical serological determination increases the evidence of previous contact with N. meningitidis antigens by 12%.