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 cur…