Study of Mast Cell Precursors

This study will investigate mast cell precursors that circulate in the blood. In a group of diseases collectively known as mastocytosis, mast cells accumulate in abnormal amounts in the skin, lymphoid tissues, bone marrow, liver and spleen. Some forms of mastocytosis have a generally good prognosis; for others, the prognosis is poorer. There is no known cure for any form of the disease. A better understanding of mast cells and how they respond to certain substances may provide insights that will lead to effective treatments for mastocytosis. Patients with systemic mastocytosis and normal healthy volunteers between the ages of 20 and 60 may be eligible for this 8-day study. Participants will undergo the following procedures: * Day 1 Medical history, physical examination, and blood tests to assess general health status * Days 2 through 6 Daily injections under the skin of G-CSF a hormone that stimulates white blood cell production * Day 7 Leukapheresis a procedure for collecting large numbers of white blood cells. In leukapheresis, blood is drawn through a needle placed in an arm and channeled into a cell separator machine. The white cells are collected and the rest of the blood is returned to the body through a needle in the other arm. The procedure takes up to 3 hours. * Days 7 and 8 Blood draw (about 1 teaspoon) to monitor white blood cell counts.