A food allergy is an abnormal response by the body to a food which is usually “harmless” and tolerated by the vast majority of the population. The food which the patient reacts to is termed an “allergen.” In people with a food allergy, the immune system “misjudges” the allergen to be unsafe and sets up a response to the allergen, similar to the body’s response to a parasite. This results in a variety of symptoms.

Certain allergens such as cow’s milk and hen’s egg are universally common, and others such as peanut are increasing in many parts of the world. Other common food allergens in children are wheat, soya, fish and tree nuts. Some parts of the world have a higher rate of other more unusual allergies- such as mustard allergy in France and sesame seed allergy in Israel.

Adults have a slightly different list of most common allergies, as many have outgrown milk, egg or wheat allergies. In adults, nut and seafood allergies are the most common offenders. Certain fruits or vegetables may also start causing issues in older children or adults who have previously suffered from a pollen allergy- this is called the oral-allergy syndrome (also known as the pollen-fruit syndrome).

There are two main types of food allergy: the immediate type (medically termed IgE- mediated as it is directed by the antibody type E), and the delayed type of allergy (medically termed “non-IgE mediated” as it involves different mechanisms).

By definition, immediate type allergies cause symptoms which start soon after ingesting the offending food: within seconds to 2 hours after. Symptoms usually last only a few hours but in some cases a prolonged reaction may occur or a flare up of the symptoms can recur a few hours later.

Typical symptoms of an immediate-type reaction include :

• Itchy rashes- classically hives
• Swelling, for example of the lips or eyes
• Itchy eyes and a runny nose
• Itching in the throat or palate
• Abdominal pain, nausea or vomiting
• Cramps and diarrhoea
• An irritation in the throat or persistent cough
• Swelling of the upper airway causing noisy croupy breathing
• Blockage of the lower airways causing an asthma-like wheeze or tight cough
• Feeling dizzy or faint as a result of a drop in blood pressure
• Feeling anxious and irritable

Skin rashes or swelling, or abdominal symptoms on their own make up a milder reaction; if different systems are involved (for example skin and gut) this is a more severe reaction. If there is any involvement of the respiratory system (coughing, wheezing, breathing difficulties) or cardiovascular system (collapsing, or a drop in blood pressure) this is a very severe reaction termed “anaphylaxis.” Severe reactions and anaphylaxis require urgent treatment as they can be life-threatening.

In delayed type food allergies, there is a time lapse of hours to days after ingestion of the offending food before the body shows signs of reacting. Sometimes, the food needs to be ingested regularly for a while before the body reacts. Delayed type allergies commonly present in babies and young infants, although specific types (such as the condition called eosinophilic oesophagitis) may present in later childhood or even in adults. Most commonly they present with gut and/or skin symptoms. Although these allergies may be serious, they are not immediately life threatening such as anaphylaxis. Cow’s milk and soya are by far the most common offenders causing delayed type allergies in children, although in theory any food protein may contribute.

Symptoms of delayed type allergies include :

• Recurrent gut cramps such as severe colic (note: most cases of “baby colic” are not as a result of milk allergy! In the severe end of the spectrum, or in those with other symptoms such as rashes, allergy should be considered)
• In some cases, severe reflux, especially that which is not responding to standard treatment
• Recurrent vomiting or diarrhoea
• Blood or mucous in the stools
• Irritability and poor feeding may result
• Chronic constipation not responding to regular treatment
• Eczema-like rashes may occur: note that most cases of eczema are not due to food allergy, but eczema not responding to treatment, or eczema together with gut symptoms, may represent a delayed type allergy.

Delayed type allergies are more difficult to diagnose because of the time delay between eating the food and symptoms.