Allergy is when the body’s immune system mistakenly responds to certain things eg peanuts, dog fur, that it thinks are harmful to the body, when they’re really not.
When this happens, things can go really wrong.
Why does it happen? We still don’t really know! The body learns to recognise things outside itself as being harmful or not harmful, and needs to do this pretty soon after birth, and somehow this goes wrong.
The job of recognising things outside the body as being harmful or not is done by special cells in the body. One kind of cell, the Antigen Presenting Cell, takes chunks of protein (“antigen”) found in the gut, but also on the skin, in the mouth, nose and lung) and present them to another kind of cell, the T-lymphocyte (these are one of the main types of white cell found in the blood). It is this T-cell that tries to recognise whether or not that protein is harmful or not. If the T-cell thinks that the antigen is something harmful, it will start a biological chain reaction. This reaction is similar whether the trigger is a bug eg a bacterium, or a food that you’re allergic to – there’s inflammation, with all kinds of chemicals and antibodies being produced.
The inflammation in allergy is typically caused by Mast cells, another cell type, found throughout the tissues of the body. Mast cells tend to sit inactive until they are triggered, at which point they release a whole mix of different chemicals into the surrounding tissue, often causing significant damage. One of these chemicals is Histamine, which causes dilation of blood vessels (producing redness and swelling) and also irritates nerve endings (producing itch). That’s why we use anti-histamines to treat or prevent allergic reactions.