Why do people have allergies?

On first exposure to an allergen, a sensitised immune system produces antibodies called immunoglobulin E, or IgE. These antibodies attach to mast cells in the skin, airways and gut. On later exposures, the allergen binds to the IgE and the mast cells release histamine and other chemicals, causing sneezing, itching, swelling or hives. Genetics play a large role, and children with allergic parents are more likely to develop allergies themselves. The hygiene hypothesis suggests that reduced early exposure to microbes may partly explain why allergies have become more common in industrialised countries.