CD19 and Immunodeficiency: Depletion of normal CD19-expressing B cells due to “on-target, off-tumor” effects of CD19-directed CAR T cells may lead to prolonged B cell aplasia and profound immune deficiency after CAR T cell therapy, thereby posing a unique challenge for both acute and long-term prevention of infections [106,107]; select patients may benefit from immunoglobulin prophylaxis.