Proposed mechanisms of increased calcium mobilization induced by anti-B4–bR. (1). Homodimerization of monomeric anti-B4–bR occurs either in solution or on cell surface, leading to cross-linking of two CD19 molecules. (2a). The MoAb moiety of anti-B4–bR binds to one CD19 antigen, while a putative residual galactose binding site within the blocked ricin moiety binds to cell surface molecules. (2b). The latter interaction is specifically inhibitable with lactose. It remains to be established whether the surface molecules, which bind to blocked ricin, are CD19 or other molecules that synergize with CD19 signaling. In both models, the downstream signaling pathways may include promotion of physical interaction between CD19 and receptor-associated PTK, followed by phosphorylation of PLCγ, generation of IP3, release of calcium from intracellular stores, and influx from the extracellular milieu. The two models proposed are nonexclusive and could be complementary.