Tissue mast cells and the closely related blood basophils are distinguished by their special mediators and their high-affinity IgE membrane receptors. These properties have made the cells be viewed traditionally as effector cells of immediate-type hypersensitivity reactions. More recently, mast cells have been shown to develop from the myeloid series of the bone marrow under the influence of interleukins 3, 4, 6, 9, 10 and 11. Mast cell and basophil lines of murine and human origin have also been shown to express and release interleukins 1, 3, 4, 5, 6, 8, tumor necrosis factor-α, granulocyte-macrophage colony-stimulizing factor and interferon-γ. Certain cytokines are also able to induce mediator release from mast cells. Furthermore, mast cells have in the past been shown to be modestly phagocytic and to express major histocompatibility complex class II membrane markers after appropriate stimulation. These findings show that mast cells are intricately associated with immune reactions and suggest that they may not only sustain and modulate but possibly also initiate immune response