B Cell Aplasia Following CAR T Cell Therapy: Incidence, Kinetics, Prognostic Implications, and Clinical Management

B cell aplasia (BCA), the sustained depletion of circulating CD19-positive B cells, is the defining on-target, off-tumor consequence of anti-CD19 chimeric antigen receptor (CAR) T cell therapy. Despite its occurrence across all approved CAR T cell products and all responding patients, BCA has not been systematically reviewed as a standalone clinical and biological phenomenon. This review synthesizes data from landmark trials and real-world cohorts across B cell-acute lymphoblastic leukemia (B-ALL), large B cell lymphoma, follicular lymphoma, mantle cell lymphoma, and chronic lymphocytic leukemia to characterize BCA incidence, recovery kinetics, prognostic significance, and management implications. Anti-BCMA products, which mechanism of action differs fundamentally, depleting plasma cells rather than B cell precursors, are intentionally excluded from this review. BCA is observed in all responders and is consistently absent in non-responders across reported cohorts , making it a reliable pharmacodynamic marker of CAR T cell activity. Its prognostic significance is disease-specific: in lymphoma, BCA recovery does not predict relapse and durable remission is achievable independent of sustained aplasia; in B-ALL, early BCA recovery within six months is a robust independent predictor of CD19-positive relapse, while persistent BCA correlates with sustained remission. CD19-negative antigen-escape relapse occurs preferentially in the presence of intact BCA and high pre-infusion tumor burden. Combining BCA kinetics with bone marrow next-generation sequencing minimal residual disease assessment at day 28 and month 3 constitutes the most powerful post-infusion risk-stratification framework currently available. BCA is mechanistically dissociated from hypogammaglobulinemia: IgM declines rapidly and profoundly, IgA more slowly, while IgG-maintained by CD19-negative long-lived plasma cells-is the most preserved isotype. No B cell count threshold below which hypogammaglobulinemia becomes clinically significant has been established. Evidence-informed IVIG replacement thresholds are proposed, though no randomized trial data exist to support them, representing a critical gap requiring prospective investigation.