A Study to Assess the Safety and Effects of ABBV-1758 Following Subcutaneous or Intravenous Injections in Participants With Alzheimer's Disease

Inclusion Criteria: * Participants meeting all the following criteria for Alzheimer's disease (AD): * In regions where timely testing is feasible (e.g., results available within 4 weeks of Visit 1), plasma biomarker that is predictive of elevated brain amyloid at Screening for participants that do not have known elevated brain amyloid based on previous amyloid positron emission tomography (PET) results. * Participants with amyloid positron emission tomography PET scan results consistent with significant amyloid pathology (as determined by a Centiloid value of 50 or higher). * Participants must have a Mini-Mental State Examination (MMSE) score of 20 or higher at Screening. Exclusion Criteria: * Participants with screening magnetic resonance imaging (MRI) that show evidence of another potential etiology for progressive dementia. * Participants who have any current serious conditions or illnesses that are not adequately controlled, or any conditions that, in the investigator's opinion, could interfere with the analyses in this study, including but not limited to psychiatric, neurologic (other than AD), cardiovascular, hepatic, renal, gastroenterological, respiratory, endocrinologic, immunologic, or hematologic, metabolic, pulmonary, ophthalmologic, dermatologic, and/or any history of abnormal laboratory results that are indicative of significant disease(s). * Participants who had prior exposure to ABBV-1758 or any history of exposure to anti-amyloid beta monoclonal antibody (mAb) treatment. * Participants with other significant pathological findings on brain MRI at screening, including but not limited to: * Evidence of vasogenic edema * 4 or more microhemorrhages (defined as 10 mm or less at the greatest diameter) * Any macrohemorrhage (defined as greater than 10 mm at the greatest diameter) * Any superficial siderosis * Severe white matter disease