Tournilhac et al. demonstrated that the human mast cell line HMC-1 stimulated proliferation of the malignant lymphoplasmacytic cells of patients with Waldenstrom's macroglobulinemia through interactions between CD154 on the mast cells and CD40 on the lymphoplasmacytic cells [13]. In vitro studies also suggest a role of mast cells in the growth of Hodgkin lymphoma via their expression of CD30 ligand [14]. Here, CD40 is linked to Waldenstrom macroglobulinemia.