Type I mAbs internalize in primary normal and malignant human B cells. (A) Normal human peripheral blood B cells were isolated by negative selection, treated with Ritux-488 or Tosit-488 (5 μg/mL) for 2, 6, or 24 hours, and then assessed for internalization as before. Results from 8 different donors are shown. *At each time point, the level of surface-accessible CD20 with Ritux-488 was significantly lower than with Tosit-488 (P < .001). (B) CLL, MCL, FL, or DLBCL samples were treated with Ritux-488 or Tosit-488 (5 μg/mL) for 2 or 6 hours and then assessed for internalization as before. The amount of surface-accessible CD20 after Ritux-488 was significantly lower than after Tosit-488 at 6 hours for CLL, MCL, and FL (P < .001, P = .001, and P < .001, respectively). The table indicates the significance of the difference in the rate of internalization of Ritux-488 between CLL and other disease subtypes. Each data point represents a sample from a different patient: CLL, n = 26; MCL, n = 5; FL, n = 9; and DLBCL, n = 5.