Mast cells (MC) are key regulators of chronic skin inflammation, such as atopic eczema, and can also give rise to a group of diseases called mastocytosis. How MC numbers are regulated in these conditions is poorly understood. We have identified a novel circulating precursor cell that gives rise to MC. We will determine the function of these precursors in skin diseases, including eczema and mastocytosis, with the aim to curtail the course of of these difficult-to-treat conditions.