Testing Experimental Alzheimer's Disease Medication

Eligibility Details: Inclusion Criteria: 1. Total body weight between 50 and 120 kg, inclusive. 2. Clinical laboratory evaluations (including chemistry panel fasted [fasted at least 8 hours], complete blood count (CBC), and urine analysis) within the reference range for the test laboratory, unless deemed not clinically significant by the Investigator. A count of the segmented neutrophils and bands should be performed when results from the white blood cells (WBCs) are not within the reference range. 3. Negative test for selected drugs of abuse at screening (does not include alcohol) and at admission (testing at admission does include alcohol breath test). A positive result may be verified by re-testing (up to one false positive result permitted) and may be followed up at the discretion of the Investigator. 4. Females must be non-pregnant and non-lactating, and either surgically sterile 5. In good health, determined by no clinically significant findings from medical history, physical examination, 12-lead electrocardiogram (ECG), laboratory tests, and vital signs. For MD cohort 1. Ages 50-85 years, inclusive. 2. The participant should be capable of completing assessments either alone or with the help of the study partner (where appropriate), per local guidelines. 3. Availability of a person ("study partner") who, in the Investigator's judgment, has frequent and sufficient contact with the participant and is able to provide accurate information regarding the participant's cognitive and functional abilities, agrees to provide information at clinic visits, which require partner input for scale completion, and signs the necessary consent form, per local guidelines. 4. Clinical diagnosis of probable Alzheimer's disease dementia based on National Institute on Aging Alzheimer's Association criteria. Exclusion Criteria: 1. Pregnant or lactating, or intending to become pregnant within 16 weeks after last dose of study drug. 2. Participation in a clinical trial within 30 days before randomization; use of any experimental oral therapy within 30 days or 5 half-lives prior to Day 1, whichever is greater; or use of any biologic therapy within 12 weeks or 5 half-lives prior to Day 1, whichever is greater. Participants who have received an experimental therapy that has no half-life, like a vaccine, should have completed that therapy at least 12 weeks prior to Day 1. Participants who have received an experimental vaccine against a central nervous system (CNS) target, such as beta-amyloid or tau, are not eligible for this study. 3. Any non-experimental vaccine within 2 weeks of randomization, until 2 weeks after the last dose. It is advised that prospective participants receive their annual influenza vaccine as early as possible in advance of the flu season, and then wait 2 weeks prior to randomization. It is permitted to receive the annual influenza vaccine during the screening period. 4. Surgery or hospitalization during the 4 weeks prior to screening. 5. Planned procedure or surgery during the study. 6. Systemically, clinically significantly immunocompromised patients, owing to continuing effects of immune suppressing medication. 7. Known history of severe allergic, anaphylactic, or other hypersensitivity reactions to chimeric, human, or humanized antibodies or fusion proteins. 8. Past history of seizures, with the exception of childhood febrile seizures.