Restoring CD20 expression in Raji cells within 24 hours after exposure to rituximab. Rituximab and plasma were first added to Raji cells and the CD20 was masked (A) and rituximab was detected on the surface of cells (B). After 2 hours of incubation with plasma from a healthy individual, the CD20 was down-modulated (C) and a low level of rituximab was detected on the surface of cells (D). Cells were washed and kept in culture media. After 24 and 48 hours, most cells became positive for CD20 (E,G). Rituximab was detectable on the surface of cells in some cells (F,H). The percentage of cells in each quadrant is shown.