occur when the body's immune system reacts to a particular substance as though it's harmful. It's not clear why this happens, but most people affected have a family history of allergies or have closely related conditions, such as asthma or eczema.
The number of people with allergies is increasing every year. The reasons for this are not understood, but 1 of the main theories is it's the result of living in a cleaner, germ-free environment, which reduces the number of germs our immune system has to deal with. It's thought this may cause it to overreact when it comes into contact with harmless substances.