Extensive crosslinking of CD22 by plate-immobilized epratuzumab induced intracellular changes in Daudi cells similar to ligating B-cell antigen receptor (BCR) with a sufficiently high amount of anti-IgM. Either treatment leads to phosphorylation of CD22, CD79a and CD79b, along with their translocation to lipid rafts, both of which were needed to induce caspase-dependent apoptosis. Immobilization also induced stabilization of F-actin, phosphorylation of Lyn, ERKs and JNKs, generation of reactive oxygen species (ROS), decrease in mitochondria membrane potential (Δψm), upregulation of pro-apoptotic Bax, and downregulation of anti-apoptotic Bcl-xl and Mcl-1. Several of the in vitro effects of immobilized epratuzumab, including apoptosis, drop in Δψm, and generation of ROS, were observed with soluble epratuzumab in Daudi cells co-cultivated with human umbilical vein endothelial cells. The in vivo mechanism of non-ligand-blocking epratuzumab may, in part, involve the unmasking of CD22 to facilitate the trans-interaction of B cells with vascular endothelium.