H1-0 and leukemia: To demonstrate the pathophysiological roles of histone H1 on mast cell-mediated allergic responses in vitro, rat basophilic leukemia cell line RBL-2H3 were sensitized with anti-DNP IgE and then stimulated with calf thymus histone H1 (25, 50 and 100 μg/ml) in the absence of DNP-BSA.