How long does it take for food allergies to occur?

The onset time of most food allergies is concentrated within a few minutes to 2 hours after eating, and only a small number of delayed-type allergies will occur several hours to 2-3 days after eating.

When I was doing follow-up registration in the allergy department, I met many patients with immediate allergies. The one that impressed me most was a junior high school boy who was allergic to mango. Last time, his classmate handed him a mouthful of mango pancake. Before he could swallow it, his lips swelled into "sausage mouth" in less than 3 minutes. This type of allergic reaction is mostly IgE-mediated. After the allergen enters the body, it will directly combine with the pre-existing antibodies. The immune system instantly releases a large amount of inflammatory mediators such as histamine. It is like pressing the emergency button of a fire alarm. The alarm will sound immediately. The attack speed is very fast. In severe cases, laryngeal edema and dyspnea may occur in more than ten minutes. You must go to the doctor immediately.

However, not all food allergies are so "blatant". The onset of many delayed-type allergies can be so slow that you would never think it is related to diet. Previously, a mother brought her two-year-old child to check the cause of recurrent diarrhea. After checking for infection and digestive enzymes, there were no problems. Finally, she made a food diary and discovered that the child would start to have watery stools in about 2 days after eating complementary food with cod. I had never thought about allergies before, after all, it had been so long. This type of delayed allergy is not IgE-mediated. The immune system needs to slowly accumulate the amount of allergen stimulation. Symptoms will not appear until the tolerance threshold is exceeded. It is like pouring water into a cup and it will not overflow until the last sip. Many people are not aware that it is related to the diet a few days ago, and it is also troublesome to check.

Nowadays, many science publications say that food allergies will attack within 24 hours. In fact, this statement is for immediate allergies, which account for more than 90%. There are indeed clinical cases of delayed allergies that take more than 72 hours to occur, but the proportion is very low. Many ordinary sciences will not mention it specifically, which has also caused a lot of cognitive bias. If you eat new food that you have never been exposed to before, and you develop itchy skin, rashes, numb lips, and chest tightness within a few hours, it is most likely a food allergy. Don’t force yourself to go see a doctor immediately. ； If you have unexplained recurring rashes or diarrhea, you can also review the food records of the past three days, and you may be able to find deeply hidden allergens.