This study will 1) identify characteristics of bone marrow mast cells in patients with unexplained anaphylaxis and flushing or with mastocytosis and 2) determine whether mastocytosis might be the underlying cause of unexplained anaphylaxis in some patients with this condition. Anaphylaxis is a hypersensitivity reaction in which patients may have flushing, hives, stuffy nose, red itchy eyes, difficulty breathing, swelling of the tongue, throat, palms and soles, abdominal cramping, lightheadedness, decreased blood pressure, and loss of consciousness. Although allergens are a common cause of anaphylactic episodes, no cause can be identified in up to 50 percent of patients who have recurrent events. Mastocytosis is a disease of excessive mast cells in tissues such as skin and bone marrow. These cells can release chemicals that result in itching, blisters, flushing, bone pain, and abdominal pain.