Background: * Omalizumab is an approved drug for the treatment of asthma by the Food and Drug Administration. * Researchers are now studying this drug in a double-blind placebo-controlled manner to assess efficacy in patients with idiopathic anaphylaxis (recurrent hypersensitive allergic episodes for which a cause is not identified). * The study will improve understanding of the mechanisms involved in anaphylactic reactions as a response to the downregulation (a decrease in the number of receptors on the surface of cells) in mast cell (a resident cell with several types of tissues) activation, and lead to the development of strategies to better prevent or treat anaphylaxis. Objectives: * To determine whether treatment with omalizumab will reduce or prevent episodes of unprovoked anaphylaxis (an acute allergic reaction) in subjects with a history of idiopathic anaphylaxis. * To assess pharmacodynamics (physiological effects of a drug) and identify patients with undiagnosed mastocytosis (rare disorders caused by too many mast cells). * To investigate cellular and molecular mechanisms of signaling and the effect of omalizumab on mast cells or basophils (a cell in the leukocyte family that releases histamine, which affects allergic response) and explore other regulatory pathways that may be involved with modulation of mast cell degranulation. Eligibility: * Patients between 18 and 70 years of age who have been diagnosed with idiopathic anaphylaxis, a diagnosis that is made only after other causes of anaphylaxis have been considered. * Patients with documented anaphylaxis episodes (mild to severe) at least six times within the past 1 year period, at least once within the last 4 months, and with at least one of the following: * Elevated serum tryptase above baseline within 2 hours of the event. * Emergency room visit with documented anaphylaxis without a known cause established by the acute onset of an illness (minutes to several hours) with involvement of the skin, mucosal tissue, or both (generalized hives, itching or flushing, swollen lips-tongue-throat) and at least one of the following: (1) respiratory compromise or gastrointestinal involvement (shortness of breath, wheeze-bronchospasm, throat tightness, low oxygen levels, nausea, vomiting, or abdominal pain); or (2) reduced blood pressure or associated symptoms of end-organ dysfunction (collapse, loss of consciousness, or loss of bladder or bowel control). * Hospitalization for anaphylaxis. * Patients must provide a letter of referral, with copies of pertinent medical history and laboratory tests, from the prospective participant s local physician, and have the ability to give informed consent. * Women with childbearing potential must have a negative pregnancy test, and must agree to practice abstinence or effective birth control from the start of the protocol and for 3 months following the last injection of the study drug. Design: * Participants will undergo a clinical evaluation, blood tests, and a bone marrow biopsy and aspirate. * Participants will be randomized to either drug or placebo and will receive two doses of omalizumab or a matched placebo while hospitalized, followed by continued outpatient therapy, every 2 to 4 weeks, for up to 6 months. * Participants will remain on the assigned regimen for 6 months or until they have experienced new onset of severe adverse event on one occasion within 24 hours of study medication that are related to the study drug, whichever comes first. At that time, the participant will be discontinued from drug administration.
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Reduction in the Number and Timing of Anaphylactic Events in Subjects With a History of Frequent Idiopathic Anaphylaxis.
Timeframe: 6 months