Food allergy refers to the condition when the human body has ingested certain harmless foods and mistakenly treated certain proteins in food as harmful substances. The reaction incorrectly activates the defence mechanism of the immune system, causing immediate, serious or even life-threatening symptoms (such as serious peanut allergy) or other chronic symptoms (such as gluten allergy, also known as celiac disease). Skin rashes, runny nose, itchy skin, vomiting, etc. are common symptoms of allergy. In severe situations, it may induce dyspnoea, the dropping of blood pressure, and even death. Allergic reactions are repetitive, and the same symptoms will be induced whenever the same foods are eaten.

Some common food allergens include milk, eggs, wheat, soybeans, nuts and peanuts, fish, shellfish and crustaceans. Children with food allergies are generally allergic to foods such as wheat, eggs and soybeans. However, not every food allergy reaction will improve with age. For examples, foods such as fish or peanuts will pose long-term threats to patients who are allergic to them. Besides, certain foods may suddenly become allergens to certain people, even if such foods had not posed any allergic reactions or caused any problems in those people during their childhood.

After ingestion, food will be digested and absorbed by the stomach before it reaches the immune system. The severity of food allergy depends on the amount of food, rate of absorption, and cooking methods. Meanwhile, food allergy is mainly divided into two types, namely acute allergy and chronic allergy.

Acute allergy can lead to allergic reactions within a few minutes after food ingestion, which makes it easier for patients to identify allergens. Allergic symptoms include runny nose, itchiness, skin rashes, vomiting, diarrhoea, etc. In the worst case, allergy can lead to anaphylactic shock, in which allergens trigger cells to release inflammatory factors and result in dilation of capillaries, thereby inducing a sharp drop in blood pressure and contraction of the trachea that may lead to shock or even sudden death.

Chronic allergic reactions can be delayed until hours to days after eating allergenic foods. Such reactions may lead to long-term lethargy, migraine, gastrointestinal discomfort, etc. If the condition becomes severe, the allergy may cause asthma or eczema. It is more difficult to identify allergens with chronic allergy. If necessary, patients can try to record their eating habits and allergic reactions in diaries to help doctors to follow up their conditions.

Food allergy is an adverse food reaction related to the immune system. The main reason is that the immune system has activated the defence mechanism by mistakenly treated the food as harmful to the human body, leading to allergic reactions of the body.

There are more cases of food allergy in young children than adults. Complete avoidance of any food types for children with food allergies before seeking advice from health care professionals is not recommended, as it may result in depletion of certain essential nutrients in the body. Patients can consult registered dietitians for identifying foods in their daily diet that cause allergy. They can then replace such foods with others to ensure that the body can absorb sufficient essential nutrients. The dietitians will also deliver guidance on ingredients on food labels so that patients can pay more attention when purchasing food as well as be more cautious when dining out.

Some common tests used to diagnose food allergies include skin prick tests and blood tests. When performing skin prick tests, the doctors will put some drops of allergen concentrates that need to be tested on the patients' skin and use needles to gently prick the spots covered by the concentrates. If there is an allergy, the patients' skin will show redness and swelling reactions to the allergens in about 15 minutes. For blood tests, the blood samples will mainly be sent to laboratories for measuring the level of IgE in the blood. The above tests can all help the doctors to conduct further clinical assessments on the patients, as positive results from the test do not necessarily mean that the specific allergens are associated with the clinical symptoms.