Causes Make an Appointment Refer a Patient Ask a Question Reviewed by the National Institute of Allergy and Infectious Diseases (July 01, 2009) Mastocytosis occurs when there are too many mast cells present in the body. Because mast cells release chemicals involved in the immune system, too many of these cells means that the body releases excess chemicals, such as histamine. This can then cause harm to other areas of the body, such as the skin and the lining of the stomach or intestine. Symptoms Clinical Trials For more than 100 years, National Jewish Health has been committed to finding new treatments and cures for diseases. Search our clinical trials.