All subjects will be screened for eligibility after providing signed informed consent. Once a subject is determined to be eligible, the subject will be vaccinated with the quadrivalent meningococcal conjugate vaccine at least two weeks prior to dosing, if applicable. A total of 7 dose cohorts will be enrolled sequentially and randomized to receive ALXN1007 or placebo. Subjects will receive ALXN1007 or placebo at the clinic and be kept at the clinic for a total of 4 days for observation and completion of the required study tests.