There are 2 main groups of food allergies, namely IgE-mediated (immediate type) food allergies, and non-IgE mediated (delayed type) food allergies. These types of allergies present quite differently as they have very different underlying mechanisms. IgE- mediated allergies are most commonly to milk, hen's egg, peanut and other nuts, fish, shellfish, soya and wheat. Non IgE- mediated allergies are most commonly to cow's milk and soya, but can occur to many other solid foods.
1. IgE-mediated (immediate type) food allergy
IgE-mediated food allergies are the most common type and present as "classical" food allergies with reactions typically within minutes to 2 hours of intake of the offending food. Different people need different amounts and forms of the food for symptoms to occur, for example in a milder allergy a person may need to eat a reasonable portion size before they have symptoms; in a severe allergy they may just need a tiny amount. In fact some people even react to the food in contact with the skin, e.g. after being kissed by someone who has just eaten the food, and some people react if the food particles are in the surrounding air, e.g. if someone is cooking using that food.
Systems which can be involved in IgE mediated allergy include:
- The skin: e.g hives ("urticaria"), itchy red rash, flushing, swelling of the deeper layers of the skin such as around the eyes ("angioedema")
- The gut: swelling of the lips or tongue, vomiting, diarrhoea, abdominal cramps
- The airway: sniffing and sneezing, swelling of the upper airway causing croup-like symptoms, spasm of the lower airways causing wheeze/asthma attack, difficulties in breathing
- The eyes: swelling, watering, itching
- The circulatory (cardiovascular ) system: quick heart beat, low blood pressure, fainting, collapse
- The central nervous system: anxiety, children can become withdrawn, feeling of "doom"
Although such symptoms typically occur soon after the food is eaten, there may also be some delayed reactions 6-12 hours later.
When the circulatory system is involved or the airway is significantly involved, the allergic reaction is severe, known as anaphylaxis (pronounced an-a-fi-LAK-sis).This can occur very quickly and is a medical emergency.
2. Non-IgE (delayed type) food allergy
In this type of food allergy, symptoms occur hours to days after eating the offending food, and therefore it is often more difficult to recognise or associate with a certain food. Symptoms commonly involve the digestive tract to cause symptoms such as diarrhoea, colicky pin, severe reflux, vomiting, blood in the stool or slow growth. Non digestive symptoms may include asthma and eczema. Remember that only a small proportion of eczema is caused directly by food allergy.
The effect of delayed type food allergy on the central nervous system (CNS) is a controversial area. CNS disorders which have been linked to food allergy include migraine, the allergic tension-fatigue syndrome and hyperactivity. Several foods have been shown to trigger migraine: chocolate, red wine, yeast extracts, hard cheeses, milk and eggs. However, these effects are more likely non-allergic (non-immune) food intolerances.
3. Cross reactivity syndromes
Being allergic to a food may also result in being allergic to a similar protein found in something else. For example, people who are tree pollen allergic may develop reactions to certain fruits or nuts. This is known as cross-reactivity. Cross-reactivity happens when the immune system thinks one protein is closely related to another. When foods are involved it is called oral allergy syndrome (OAS).