This is a randomized, open-label study in adult patients who have completed standard first line therapy for large B-cell lymphoma (LBCL) and achieved a complete response or partial response suitable for observation, but who have minimal residual disease (MRD) as detected by the Foresight CLARITY™ Investigational Use Only (IUO) MRD test, powered by PhasED-Seq™. The purpose of the trial is to assess the efficacy and safety of consolidation with cemacabtagene ansegedleucel (cema-cel), an allogeneic CD19 CAR T product, as compared to standard of care observation. In this study, participants with MRD are randomized 1:1 to treatment with cema-cel or an observation arm. Treatment includes cema-cel following a lymphodepletion regimen of fludarabine and cyclophosphamide. Prior to August 2025, participants may also have received an anti-CD52 monoclonal antibody, ALLO-647, as part of their lymphodepletion regimen.