All allergies involve the immune system and are usually more serious than food intolerance.
Symptoms of allergies are:
Running or blocked nose
Sore/red itchy eyes
In some cases only a very small amount is needed to cause anaphylaxis (severe allergic reaction which causes swelling of the airways and lowering of the blood pressure) Anaphylaxis could result in death unless an injection of adrenaline is given.
Food labels must state if a food contains nuts and even if it may contain nuts. This warns consumers that a very small amount of nut may have accidentally been in contact with there product, e.g when a food is processed in an area where nuts have previously been.
This is an intolerance to gluten. Gluten is a protein found in wheat, barely and rye. Oats contain a similar protein and may also need to be avoided by coeliacs.
Coeliacs cannot absorb nutrients if they eat gluten. Symptoms include diarrhoea and weight loss.
Coeliac disease is treated by not eating gluten.
You can buy special gluten free products.