Summary: This study will examine how mast cells (cells involved in allergic reactions) migrate and multiply in the skin of patients with mastocytosis, a condition characterized by too many mast cells in the body. The mast cells tend to multiply in the skin, causing dark, itchy skin spots known as urticaria pigmentosa. This study will determine if the skin of patients with mastocytosis produces chemicals called cytokines that cause mast cells to migrate to the skin and multiply. The findings will be compared with those from normal volunteers and patients with atopic dermatitis, a skin disease characterized by recurrent itchy rash usually seen in people with a family history of allergies.