The membrane protein CD20 of B lymphocytes is the target of Rituximab (RTX), the first successful therapeutic antibody. The biological function of CD20 was a mystery so far. We found that loss of CD20 leads to the reorganization of a protein/lipid nanodomain on the plasma membrane, the activation of human B cells, and their differentiation into antibody-producing plasma cells. These finding shed new light on the functional mem...