Inclusion Criteria:

• Healthy male or female participants, 18 to 60 years of age.
• First-generation Chinese ancestry (born in China, Hong Kong, Taiwan, or Macau with 2 Chinese parents and 4 Chinese grandparents) or first-generation Japanese ancestry (born in Japan with 2 Japanese parents and 4 Japanese grandparents).
• Body mass index (BMI) 18.0 to 30.0 kg/m^2 (inclusive) and body weight ≥40 kg.
• In good general health based on medical history, physical examination, vital signs, electrocardiogram (ECG), and clinical laboratory evaluations.
• Female participants must not be pregnant or breastfeeding.
• Participants of reproductive potential must agree to follow protocol-specified contraception requirements.
• Willing to maintain usual diet and physical activity regimen throughout study participation.

Exclusion Criteria:

• Clinically significant medical condition, disease, or abnormal finding that could interfere with study participation or interpretation of results.
• Clinically significant ECG abnormalities
• Clinically significant abnormal blood pressure or pulse rate at screening/check-in.
• History of malignancy (except adequately treated in situ cervical cancer or non-melanoma skin cancer >5 years before dosing).
• Deep vein thrombosis or pulmonary embolism within 3 months prior to check-in.
• History of hypersensitivity/anaphylaxis to biologic therapies, mammalian-derived products, or AMG 691 excipients.
• Active, chronic, recurrent, or unresolved infection; history of severe infection requiring intravenous (IV) antibiotics within the past 3 years.
• Positive or indeterminate QuantiFERON-TB Gold test.
• Untreated or unresolved helminth parasitic infection.
• History of immunodeficiency.
• Receipt of live or live-attenuated vaccines within 12 weeks before check-in or planned during the study.
• Estimated glomerular filtration rate (eGFR) <70 mL/min/1.73 m^2.
• Alanine Aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin, or direct bilirubin >1.5 × Upper Limit Normal (ULN).
• Positive screening tests for human immunodeficiency virus (HIV), hepatitis B, or hepatitis C infection.
• Use of prohibited prescription, over-the-counter, herbal, vitamin, or dietary supplement products within 30 days (or 5 half-lives) before enrollment, unless approved by the investigator.
• History of illicit drug use within 1 year, positive drug screen, or unwillingness to abstain from illicit drugs/cannabinoids during the study.
• Use of tobacco or nicotine-containing products within 6 months prior to check-in.
• History of alcohol abuse or excessive alcohol consumption.
• History of non-suicidal self-injury within 5 years or unstable major depressive disorder/other severe psychiatric disorder within 2 years.
• Participation in another investigational drug study within 90 days (or 5 half-lives) prior to check-in.
• Previous exposure to AMG 691.
• Recent donation of blood, plasma, or platelets.
• Any participant considered unsuitable by the investigator or unwilling to comply with study restrictions.