A Study to See How BIIB122 Works in the Human Body, and to Evaluate it's Safety, and Tolerability in Healthy Adult Japanese, Chinese, and Caucasian Participants

Key Inclusion Criteria:

• Have a body mass index (BMI) between 18 and 30 kilograms per square meter (kg/m^2), inclusive. Body weight (BW) ≥50 kg and ≤100 kg at Screening
• Negative polymerase chain reaction (PCR) test result for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at Screening and Day -1
• For Japanese healthy volunteers: Participant was born in Japan, and biological parents and grandparents were of Japanese origin; if living outside of Japan for more than 10 years, must not have significantly modified diet since leaving Japan
• For Chinese healthy volunteers: Participant was born in China, and biological parents and grandparents were of Chinese origin; if living outside of China for more than 5 years, must not have had a significantly modified diet since leaving China. Additionally, Chinese healthy volunteers must be of the same gender and have a screening weight within ±15% of their matched Caucasian healthy volunteer
• For Caucasian healthy volunteers: Participant must be a white person of European descent, which may include participants of Hispanic descent. Additionally, Caucasian healthy volunteers must be of the same gender and have a screening weight within ±15% of their matched Japanese healthy volunteer

Key Exclusion Criteria:

• History of any clinically significant cardiac, endocrinologic, hematologic, hepatic, immunologic, infectious, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, and renal, or other major disease, as determined by the Investigator
• History of severe allergic or anaphylactic reactions, or of any allergic reactions that in the opinion of the Investigator are likely to be exacerbated by any component of the study treatment
• Current enrollment in any other drug, biological, device, or clinical study, or treatment with an investigational drug or approved therapy for investigational use within 30 days prior to Day -1, or 5 half-lives, whichever is longer
• Immunization or vaccinations are not allowed from Screening to safety follow-up (SFU)/ early termination (ET) Visit

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.