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These are a result of an overreaction of the immune system to a harmless antigen, as in asthma, hay fever and eczema.
They are caused by allergens - for example, pollen, dust, particles of animal skin, dustmites and their faeces.
When these allergens are inhaled, the immune system recognises them as foreign and B cells produce antibodies. These antibodies coat the surface of mast cells, which line the airways, sensitising the body to the allergen.
When the allergen is encountered for a second time, it binds to the antibodies on the mast cells and stimulates them to release histamine. This enables white blood cells and fluid to leak from capillaries resulting in inflammation.
In hay fever, the inflammation occurs in the eyes, nose and throat but is not fatal and only usually occurs from May until September.
Asthma can be much more severe and over one thousand people in the UK die from it every year. During an attack, fluid and mucus collects in the airways, blocking the smaller ones. Muscles in the trachea, bronchi and bronchiole walls contract, and breathing becomes difficult. A vaccine, which will desensitise sufferers is in development, as one in seven children have asthma it could make a huge difference to lots of peoples lives.