Allergy refers to abnormal reactions of theimmune system to otherwise harmless substances, such as bacteria or viruses, butdoes not react to non-infectious environmental antigens. In people with allergies, however, the immunesystem also reacts to these substances, producing allergic reactions. Such substances, called allergens, can comefrom the patient’s natural environment, foods, medications, latex products, or insectbites. Most allergies are mediated by a class ofantibody called immunoglobulin E, IgE. IgE is produced when the body is first exposedto an allergen. Production of IgE is activated by a subtypeof T-lymphocytes, known as type 2 helper T-cells, TH2. IgE molecules then bind to their receptorson the surface of mast cells and basophils, triggering a signalingcascade that induces the release of histamine and other inflammatory chemicals. Stimulation of sensory nerves, smooth musclespasms, and are responsible for allergic symptoms, which can range from mild to severe. Mild symptoms usually consist of watery eyes,runny nose, sneezing and a mild rash; while severe reactions may include swelling, hives,difficulty breathing due to bronchospasm, and digestive problems due to increased gastrointestinalmotility. When released systemically, these chemicalscan cause extensive vasodilation and smooth muscle spasms which may lead to anaphylaxis,a life-threatening condition in which blood pressure drops and airways narrow to a dangerouslevel. The reactions are immediate, within minutesof contact with the allergen. There is also a late phase response, due tosubsequent tissue infiltration with eosinophils and other inflammatory cells. People who are sensitized to a specific allergenmay also react to other substances that contain similar antigens. This is called cross-reactivity. For example, people who are allergic to birchpollen may also have reactions to certain fruits and vegetables such as apples or potatoes,consumption of which can cause itching and swelling of the lips and oral cavity. Both genetic and environmental factors contributeto the development of allergic diseases. Allergies tend to run in families. What is inherited is the susceptibility toallergic reactions, due to irregularities in the makeup of the immune system. Early childhood exposures to bacterial andviral infections are thought to suppress TH2 cells and are therefore protective againstallergic diseases. While still a hypothesis, it does partly explainthe higher prevalence of allergies in developed countries. Other risks factors include exposure to allergensand stress. Diagnosis is usually based on symptoms andpatient’s history. A blood test, called allergen-specific serumIgE test, can also be performed. In this case, patient’s blood sample containingIgE is tested for binding to common allergens. If binding occurs, the person is allergicto that allergen. Antihistamines are effective for treatmentof mild allergies. Severe reactions require immediate injectionof epinephrine. The best way to prevent allergies is to avoidthe offending allergens. People with serious reactions to unavoidableallergens may benefit from immunotherapy. In immunotherapy, patients are injected weeklywith gradually increasing doses of the allergen, starting with a tiny amount.