Xencor, the Sponsor, is developing XmAb®5871 for diseases caused by an overreaction of the immune system (e.g., rheumatoid arthritis). This study is being carried out for them by Quintiles. Its purpose is to investigate how the body handles and responds to single intravenous (injection into a vein) doses of XmAb®5871. In addition to safety and tolerability, drug concentrations will be measured in blood sampled at intervals after its administration. Approximately 48 volunteers will be allocated to 7 groups. Groups 1 and 2 consist of 4 and, 3-7 of 8 volunteers each. After explaining the study, consenting subjects will undergo screening (medical history, examination and routine safety tests). Eligible subjects will be admitted to Quintiles before first dosing (Day –1). On Day 1 intravenous fine plastic tubes are inserted into a vein in each arm, one for blood sampling and the other to administer XmAb5871 or placebo. The starting dose is 0.03 mg/kg, which is much less than expected to have any clinical effect. Doses given to subsequent groups will be based on the earlier groups after the safety data from the previous dose have been reviewed. Subjects will only participate in one group at one dose level, and will be randomly allocated to XmAb5871 or placebo. They will not know whether they are receiving XmAb5871 or placebo. In addition to standard safety tests 24 hours after dosing, tetanus toxoid and another standard immunological challenge (KLH) will be administered to establish whether XmAb5871 influences the response of the body to these antigens (antigens are proteins that provoke an immune response consisting of production of antibodies). Subjects will be discharged on Day 4 and return for outpatient visits for up to 18 weeks depending on dose or even longer depending on the results of the blood tests.