A 23-year-old patient is hospitalized with meningitis caused…

A 23-yeаr-оld pаtient is hоspitаlized with meningitis caused by Neisseria meningitidis. Review оf the medical record reveals a previous episode of meningococcal meningitis at age 19. The patient has otherwise been healthy and has no history of recurrent respiratory or opportunistic infections. Which immune defect should the provider most strongly suspect?