Anaphylaxis is a severe systemic hypersensitivity reaction caused by release of mediators from mast cells and basophils. Signs and symptoms of anaphylaxis may include generalized flushing, hives, stuffy nose, red and itchy eyes, difficulty in breathing, swelling of the tongue, throat, palms and soles, gastrointestinal cramping, lightheadedness and loss of consciousness. Although most common causes of anaphylaxis include foods, drugs and stinging insects, a causative factor is not identified in up to 50% of the patients with recurrent anaphylactic episodes. Mast cells are important mediators of anaphylaxis. This study will examine the hypothesis that some patients with unexplained anaphylaxis may have systemic mastocytosis, a disorder of pathologic mast cell proliferation, as an underlying cause. In addition, it will investigate hematopoietic cells, which result in their tissue mast cells being more susceptible to degranulation. The study will enroll patients with mastocytosis with or without flushing or anaphylaxis, and patients with unexplained flushing or anaphylaxis. Patients will receive a clinical evaluation, blood tests and a bone marrow biopsy and aspirate. Research studies planned include flow cytometric analysis of blood and bone marrow cells, investigation of mutations or polymorphisms in genes involved in mast cell growth and differentiation, determination of surrogate disease markers and examination of mast cell growth and function. The study will improve the understanding of the mechanisms involved in anaphylactic reactions, which could in turn lead to development of strategies to better prevent or treat the episodes.