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.
