A Study of LY3439539 in Participants With Alzheimer's Disease

Inclusion Criteria: * Have gradual and progressive change in memory function reported by the participant or informant for 6 months or longer consistent with a diagnosis of dementia due to Alzheimer's disease * Have a Mini Mental Score Examination score of 16 to 24, inclusive * Meet plasma pTau criteria, as defined by the sponsor * Have body mass index within the range of 17 and 32 kilograms per square meter (kg/m²) (inclusive) at screening * Are women not of childbearing potential and men willing to practice effective contraception throughout the study. * Have adequate premorbid literacy, vision, and hearing throughout the study duration and able to complete study procedures. * Have at least 1, and up to 2, study partners who are in frequent contact with the participant (defined as at least 10 hours per week). Exclusion Criteria: * Have any contraindications for magnetic resonance imaging (MRI), including claustrophobia or the presence of contraindicated metal (ferromagnetic) implants/cardiac pacemaker. * Have a screening MRI that shows evidence of significant abnormality. * Have history of suicidal behavior, or a lifetime history of suicide attempt or acute suicidality. * Have a history of * Severe or ongoing allergy or hypersensitivity reactions * Hypersensitivity to immunizations or immunoglobulins * Two or more clinically significant or severe drug allergies, * Intolerance to topical corticosteroids or severe posttreatment hypersensitivity reactions * Have current serious or unstable illnesses, including hepatic, renal, gastroenterological, respiratory, cardiovascular, neurologic (other than AD), psychiatric, endocrinologic, immunologic, hematologic disease, or other conditions * Require treatment with another monoclonal antibody or have received treatment with biologic agents (such as monoclonal antibodies, including marketed drugs) within 3 months or 5 half-lives (whichever is longer) prior to dosing. * Have criteria that would preclude a lumbar puncture (LP). * Are currently enrolled in a clinical study involving an investigational product (IP) or any other type of medical research judged not to be scientifically or medically compatible with this study. * Have a current exposure to amyloid-targeting therapies (ATTs). Prior exposure to ATTs greater than 1 year from the last dose may be permitted at the discretion of the investigator and in consultation with the sponsor.