**Abstract**

Allergy is an immune disorder due to over responsiveness of immune system to a relatively normal and harmless antigen; derived from environmental and dietary substances commonly referred as allergens. Allergy is an IgE mediated type I hypersensitivity which is characterized by the degranulation of specialized white blood cells known as mast cells and basophils. Majority of characterized allergens are proteinaceous in nature and induce Th2 response. Specific Th2 cytokines elicit the induction of allergen specific IgE antibodies in sensitive individuals. The IgE binds to Fc epsilon receptor on basophil/mast cells and on exposure, allergens cross links the IgE and induce release of hypersensitivity mediators that result in allergic symptoms. The symptoms varies from mild allergies like hay fever, itchiness, rashes, rhinatisis, conjunctivitis to a severe condition such as Asthma and some time life threatening anaphylaxis. At present a various blood based test exist to diagnose allergies which include skin prick, patch test and Specific IgE tests. The best treatment available is to avoid exposure to allergens alternatively use of anti-histamines, steroids or other symptom reducing medications are in practice. Immunotherapy to desensitize the response to allergen and targeted therapy are promising for allergy in future.

**Keywords:** allergy, Th2 cells, Immunoglobulin E, Basophils, Mast cells, Histamine
