True food allergy

In children, common allergy-provoking foods include cow's milk protein, egg white from hens, wheat, soya bean, codfish and peanuts.
In adults, nuts including Brazil, almond, hazelnut, peanut and walnut are common allergens. Seafood such as fish, mussels, crab, prawn, shrimp and squid may also cause allergic reactions.

Children and food allergies
Contact your GP to discuss any suspected food allergies or intolerance's before putting a child on a restricted or elimination diet. Self-diagnosis can lead to malnutrition.
Localised oral allergies may occur in young adults in association with silver birch tree pollen allergy. They get an itchy mouth and throat on eating certain fresh fruit (apple, cherry, peach and nectarine), raw vegetables (carrot, celery and potato) and nuts.

What are the symptoms?

Typically, an immediate food allergic reaction will involve the immune system. Within minutes, traces of the offending food in the diet can trigger generalised rashes, itching, diarrhoea, vomiting, swelling of the lips and soft tissues, breathing difficulties and even shock.
Peanut anaphylaxis is a good example where traces of the food are absorbed in the mouth or intestine. This leads to the rapid release of histamine from cells and allergic tissue swelling.
Delayed reactions to food may also occur, which can aggravate eczema in infants. Coeliac disease is a delayed immune reaction to the gluten part of wheat.
This damages the intestinal lining, resulting in abdominal bloating, discomfort, diarrhoea or constipation. It also decreases absorption of essential foods from the intestine resulting in anaemia, lethargy and nutritional deficiencies. These changes may be subtle and can easily be missed.

Food intolerance reactions are of slower onset than allergic reactions, don't involve the immune system and aren't usually life threatening. They're often called 'pseudo-allergic reactions'.
Lactose intolerance, for example, is the inability to digest the cow's milk sugar lactose, caused by deficiency of the sugar-digesting enzyme lactase in the intestine.
This is common in people of southern European or African descent and results in smelly diarrhoea, pain and bloating after drinking cow's milk or taking in dairy products. Lactose intolerance doesn't cause rashes, weight gain or lethargy.
Natural histamine may be too rapidly absorbed from food in the diet and effectively lead to a histamine 'rush' with headaches, palpitations and flushing that mimics an allergy.
Then there are adverse reactions to chemical preservatives and additives in food, such as sulphites, sodium benzoate, salicylate, monosodium glutamate (MSG), caffeine and tartrazine.
These reactions are usually dose-related, with small amounts of the food being tolerated but larger amounts leading to reactions such as urticaria, flushing, abdominal pain, vomiting and diarrhoea.