What is a Food Allergy?
Food allergy is now recognised as an important food safety issue and is caused when the body mistakenly makes an antibody (IgE) to 'fight off' a specific food. When the food is next eaten (or sometimes is just in contact with the skin) it triggers an immune system response which results in the release of histamine and other substances in the body. These cause various symptoms, depending on where in the body they are released. Very rarely the immune system chemicals are released throughout the body, causing a 'systemic' reaction (such as anaphylaxis).

What Could I Be Allergic To?
You can be allergic to any food substance. Some of the more common food allergies are peanut allergy; tree nut allergy; egg allergy; milk allergy (dairy allergy); wheat allergy; fish allergy; soya allergy and sesame allergy. Some people also suffer from alcohol allergy, mustard allergy and fruit and vegetable allergies.

What Are the Symptoms of a Food Allergy?
Normally food allergy symptoms appear within a few minutes of eating the offending food, although they may be delayed by up to a couple of hours. The symptoms are usually those of 'classic' allergy, some of which are listed below:
Gut reactions - Abdominal pain, Vomiting, Diarrhoea 
Skin reactions - Itching, Swelling (rash or nettle rash)  
Respiratory reactions - Runny nose, Sneezing, Wheeze, Cough  
The greatest care must be taken by all food manufacturers either they be small or large;
- to formulate foods so as to avoid, wherever possible, inclusion of unnecessary major allergens as ingredients;
- to organise raw material supplies, production, production schedules and cleaning procedures so as to prevent cross-contact of products by "foreign" allergens;
- to train all personnel in an understanding of necessary measures and the reasons for them;
- to comply with the relevant labelling legislation providing appropriate warning, to potential purchasers, of the presence  of a major allergen in a product;
- to have in place an appropriate system for recall of any product found to contain a major allergen not indicated on the label warning.
Under Annex II of the EU Food Information for Consumers Regulation No.1169/2011 the following 14 known food allergens that must always be labelled in pre-packed and identified in some way for non-prepacked foods.
- Cereals containing gluten, namely: wheat (such as spelt and Khorasan wheat), rye, barley, oats 
- Crustaceans for example prawns, crabs, lobster, crayfish
- Eggs
- Fish
- Peanuts
- Soybeans
- Milk
- Nuts; namely almonds, hazelnuts, walnuts, cashews, pecan nuts, Brazil nuts, pistachio nuts, macadamia (or Queensland) nuts
- Celery (including celeriac)
- Mustard
- Sesame
- Sulphur dioxide/sulphites, where added and at a level above 10mg/kg in the finished product. 
- Lupin which includes lupin seeds and flour and can be found in types of bread, pastries and pasta 
- Molluscs like clams, mussels, whelks, oysters, snails and squid  
Please check the following link for further guidance
https://www.food.gov.uk/science/allergy-intolerance
What Is the Difference Between a Food Allergy and Food Intolerance?
Food allergy is quite uncommon and normally causes symptoms within a few minutes of eating the offending food or being in contact with the relevant substance.
​Food intolerance (non-allergic hypersensitivity) is much more common. The onset of symptoms is usually slower and may be delayed by many hours after eating the offending food; the symptoms may also last for many hours, even into the next day. Some common food intolerance's include lactose intolerance; gluten intolerance and histamine intolerance.