Alzheimer's disease (AD) is a progressive, irreversible neurological disorder and is the most common cause of dementia in the elderly population. Clinical symptoms of the disease may begin with occasional forgetfulness such as misplacement of items, forgetting important dates or events, and may progress to noticeable memory loss, increased confusion and agitation, and eventually, loss of independence and non-responsiveness. This study will assess how safe and effective ABBV-916 is in treating early AD. Adverse events, change in disease activity, and how ABBV-916 moves through body of participants will be assessed. ABBV-916 is an investigational drug being developed for the treatment of early AD. This study is conducted in 2 stages. Stage A is a multiple ascending dose study. There is a 1 in 4 chance that participants are assigned to receive placebo. Stage B is a proof-of-concept study. In Stage B, there is a 1 in 5 chance that participants will be assigned to receive placebo. The first 6 months of this study are double-blind, which means that neither the trial participant nor the study doctors know which treatments will be given. This will be followed by a 2-year extension period in which all participants will receive ABBV-916. Approximately 195 participants aged 50-90 years will be enrolled in about 90 sites across the world. Participants will receive intravenous (IV) doses of ABBV-916 or placebo once every 4 weeks (Q4W) for 24 weeks and will be followed for an additional 16 weeks. Participants will have the option of participating in a 2-year, open-label, Extension Period receiving IV ABBV-916. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, magnetic resonance imaging (MRI), blood tests, checking for side effects and completing questionnaires.