An open label comparison of concentration of the study medication administered intravenously (IV) versus subcutaneously (SC) in healthy volunteers.

The study will enroll approximately 50 eligible healthy male and female subjects between the ages of 18 to 55 inclusive. There will be 5 parallel dose cohorts (Cohorts 1-5) consisting of 10 subjects in each cohort. No subject will be a member of more than 1 cohort. Subjects will receive 3 administrations of study medication.

Inclusion Criteria:

• Are adult males and females aged 18 to 55 years inclusive as of dosing (Day 1) with total body weight between 45.0 and 100.0 kg inclusive and body mass index (BMI) between 19.0 and 32.0 kg/m2 inclusive;
• Healthy as assessed by the Investigator with no clinically significant abnormality identified on medical or laboratory evaluation and no history of any clinically significant disorder, condition, or disease that would pose a risk to subject safety or interfere with the study evaluation, procedures, or completion;

Exclusion Criteria:

• Subjects who have a clinically relevant history or presence of respiratory, gastrointestinal, renal, hepatic, hematological, lymphatic, neurological, cardiovascular, psychiatric, musculoskeletal, genitourinary, immunological, dermatological, connective tissue diseases or disorders that would pose a risk to subject safety or interfere with the study evaluation, procedures, or completion.
• Subjects who are positive for drugs of abuse or alcohol on screening or admission;
• Subject is pregnant or breast feeding, or planning to become pregnant within 3 months of administration of XmAb5871.
• Subjects who have used prescription drugs (with the exception of hormonal birth control for women of child-bearing potential) within 14 days or 5 half-lives, whichever is longer, prior to dosing (Day 1), unless agreed as not clinically relevant by the Principal Investigator and Sponsor.
• Subjects who have received live vaccines ≤3 months from Day 1.
• Malignancy within 5 years (except successfully treated in situ cervical cancer, resected squamous cell or basal cell carcinoma of the skin).
• Unable or unwilling to partake in follow-up assessments or required protocol procedures.