CD19 and neoplasm: While the target antigen density per tumor cell in antibody-targetable tumors has not been measured in detail, the proteins CD19 and CD20 present on B cells targeted by therapeutic antibodies and CAR-T cells are relatively well studied; for example, the antigen density of CD19 has been reported to be 2 × 104 to 3 × 104 molecules/cell in normal B cells and 0.5 × 104 to 3 × 104 molecules/cell in malignant B cells (36, 37).