The difference between food allergy and contact allergy

The core difference between food allergy and contact allergy is the different sensitization pathways - in the former, ingestion (eating, drinking) of allergens triggers an immune response, while in the latter, direct contact of the skin/mucosal membranes with allergens induces abnormal immunity. There are also significant differences in the speed of onset, predilection sites, and intervention logic between the two.

Last week, I met a 5-year-old girl in the outpatient clinic. Within ten minutes after taking a half-bite of a mango, her mouth became red and itchy. Her mother insisted that it was a contact allergy caused by the mango juice on her face. As a result, she did a food allergen test and found that the mango-specific IgE value soared to level 3. Later, the parents reported that the child would also get rashes after eating unadded dried mango, and it was not a problem of contact at all.

Many people tend to confuse the reaction caused by this kind of food on the skin with contact allergy. I usually give patients a very simple identification trick: stick peeled mango flesh on the inside of the arm for 15 minutes. If there is no reaction where it is stuck, but problems occur as soon as you eat it, then it is definitely a food allergy. Even if the reaction is the most severe where the juice touches it, it is essentially a local manifestation of a systemic reaction triggered by ingestion of allergens. On the other hand, if you just touch an unpeeled mango and your hands itch and get a rash, but nothing happens when you eat the pulp with gloves after peeling, then you have a contact allergy to the urushiol in the mango skin.

According to the current mainstream classification logic in the academic world, food allergies are mostly IgE-mediated immediate hypersensitivity reactions. In other words, the onset is quick, and symptoms will appear within a few minutes to 2 hours. They can range from wheals and swollen mouth to severe laryngeal edema and difficulty breathing, which may be life-threatening. Contact allergy is mostly a delayed type IV hypersensitivity reaction, which takes at least a few hours or even two or three days to occur after contact with the allergen. It is basically limited to the contact area, rarely spreads to the whole body, and is basically not life-threatening. Oh, by the way, there is also a cognitive difference between disciplines: many dermatologists will classify local erythema caused by food contact around the mouth as "food contact dermatitis", which is a special branch of contact allergy. Doctors in the allergology department prefer to classify it as food allergy based on the immune mechanism. In fact, both classifications are correct. The former focuses more on local skin care intervention, while the latter emphasizes comprehensive avoidance of diet. In fact, they are complementary in clinical treatment.

Take the most common metal nickel allergy as an example. Most people wear alloy necklaces and jeans with metal buckles, which cause rashes at the contact areas. This is a typical contact allergy. As long as you do not touch metal products containing nickel, you will be fine. But don’t tell me, there are a very small number of people who are allergic to ingestible nickel. They will get rashes all over their body after eating chocolate, nuts, and shellfish with high nickel content. At this time, even if they are not in contact with metals containing nickel, they will have a rash, which falls into the category of food allergies.

When I educate patients, I usually don’t ask them to memorize classifications. After all, the intervention logic of the two types of allergies is quite different: if you have a food allergy and have had a systemic reaction, it is recommended to keep an epinephrine pen with you and take it immediately if you accidentally eat it; while for contact allergies, you basically only need to avoid the source of contact and apply weak glucocorticoid ointment for three to five days and it will be cured, and systemic medication is rarely required. Of course, there are also mixed cases, such as a contact allergy to ragweed pollen and a cross-reaction with peach or apple protein. In that case, both mechanisms may be responsible for the attack, and there is no way to classify it into one category. In this case, don't make blind judgments on your own. It is best to see a doctor based on your medical history and test results.

In fact, for ordinary people, there is no need to worry about which kind of allergy it is. If something goes wrong, first clearly remember what you have eaten and touched recently, and explain it to the doctor when you see the doctor. It is much more useful than classifying it by yourself through popular science. To put it bluntly, no matter what type of allergy it is, avoiding the allergen that makes you uncomfortable is always the first priority.