A Study of MK-2214 in Adults With Mild Cognitive Impairment or Mild-to-Moderate Alzheimer's Disease (MK-2214-002)

Inclusion Criteria: * Participant is in overall good health based on medical history and laboratory safety tests * BMI between 18.5 and 35 kg/m\^2 Part 1 (MCI and Mild to Moderate AD) Only: * History of cognitive and functional decline with gradual onset and slow progression for at least one year before Screening * Have an Mini-Mental State Examination (MMSE) \>12 at the prestudy visit * Modified Hachinski Ischemic Score (MHIS) score \<4 at the prestudy visit Exclusion Criteria: * Based on clinical interview and Columbia-Suicide Severity Rating Scale (C-SSRS), has reported suicidal ideation with intent, with or without a plan or method * History of unstable or poorly controlled endocrine, gastrointestinal (GI), cardiovascular, hematological, hepatic, renal, respiratory, or genitourinary abnormalities or diseases * History of clinically significant active neurological disease (except for AD or MCI for participants in Part 1) * History of clinically significant active autoimmune disease requiring ongoing systemic immunosuppressant therapy * History of cancer (malignancy) * History of significant multiple and/or severe allergies (eg, food, drug, latex allergy), or has had an anaphylactic reaction or significant intolerability to prescription or nonprescription drugs or food * Positive test(s) for Hepatitis B Surface Antigen (HBsAg), hepatitis C antibodies or human immunodeficiency virus (HIV) * Has had a major surgery and/or donated or lost 1 unit of blood (approximately 500 mL) within 4 weeks prior to the prestudy visit * Has a contraindication to lumbar dural puncture, such as coagulopathy, concomitant anticoagulation beyond low dose aspirin, thrombocytopenia, or other factors that could preclude safe lumbar puncture * Currently receiving or has received aducanumab or another anti-amyloid therapy within the last 6 months * Has a history of receiving biological therapy within 3 months or 5 half-lives (whichever is longer) or any human immunoglobulin preparation within the last year * Has received any non-live vaccine starting from 14 days prior to first study intervention or is scheduled to receive any non-live vaccine through 14 days following the final dose of study intervention. Exception: COVID-19 and influenza vaccines may be administered * Is receiving systemic immunosuppression, including corticosteroids exceeding physiologic replacement doses