An allergy is your body's overreaction to something that usually doesn't bother most people.
For example, grasses, pollen and cat fur can cause trouble for some people.
Sometimes your skin may have red patches and feel itchy (sometimes called 'hives'). You may also feel hot and sweaty or sneezy or your eyes and nose may be sore, itchy and running (hayfever). Your throat could feel sore and froggy or you may be wheezy when you breathe, or have a cough (asthma).
There are many things that can cause an allergic reaction and reactions may range from mild to severe.