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Pathology and causes of Food Allergy

A food allergy refers to the abnormal allergic reactions to food caused by the immune system imbalance. The immune system identifies and eliminates harmful invaders such as virus and bacteria. But for those with food allergy, their immune system is so sensitive to foreign objects that it regards harmless food (usually proteins) as intruders. With the intake of an allergen, the immune system will release IgE antibodies to ‘fight against the invaders’, which will signal the mast cells to produce histamine. This will lead to allergic reactions in the skin or in the digestive and respiratory systems.

Reactions of food allergy may be immediate or delayed. Some people experience them in just a few minutes, half an hour or a day while some in a week or even two weeks’ time. For individual patients, a tiny little allergen is enough to trigger the reaction. For the even more sensitive, the smell of the allergen (e.g. steamed fish) can trigger symptoms. Allergic food reactions include swelling of the mouth, itchy or swollen skin, rashes, diarrhoea, vomiting, breathlessness, cough, or even fatal conditions such as difficulty in breathing, a rapid drop in blood pressure or in serious cases the person may go into shock.

Different food may be allergens to different individuals. But the eight most common allergens are:

  • Milk (including cheese and chocolate)
  • Eggs
  • Peanuts
  • Tree nuts (pecan, walnuts, pistachio, pine nuts)
  • Seafood
  • Shellfish
  • Soy
  • Wheat

In western countries, fish, after milk and eggs, is the third most common allergen. In China, it is seafood (including fish, shrimps, crabs, crayfish, cuttlefish, algae). In Hong Kong, 70% of those with severe food allergy (those experiencing shock) are allergic to seafood.

Other possible allergens include seeds (sesame, poppy seeds), fruits (including pineapple), vegetables, spices (including ginger), bird’s nest, royal jelly, natural and artificial colouring and food additives, and even rice. A small number of those allergic to milk, around 10%, is also allergic to beef as these two types of food have the same proteins.

Hong Kong statistics have revealed that among children aged between 2 and 7, 8.1% of their parents have seen in them adverse effect of food and 4.6% have seen a doctor for a diagnosis.

Studies have shown that the incidence rate of food allergies among Hong Kong children is about 5 to 8%. The eight most common allergens are chicken eggs, cow’s milk, crustacean seafood, fish, wheat, soy, peanuts and tree nuts. As recommended by experts, parents should not only rely on medicine to relieve eczema symptoms for their children; they need to learn more about food allergy, do a food allergy test and help them avoid such food.