I have seen a few of cases now in which the CD20 expression has been
altered, either reduced or become negative in partients with B cell
malignancy who have been treated with anti CD20 antibody. In one case the
CD20 expression was negative by flow and only weak and patchy on IHC,
perahaps sampling influencefor this discrepancy. Also one case showed
internalization of the CD20 staining.

Murray Horn
Supervising Scientist Immunohistology
QHPS_PAH Campus
Brisbane
Australia