How long does it take for a baby’s food allergy to appear?

I have talked about this issue with a pediatrician I am familiar with before. The time span for the onset of food allergies in babies is actually quite wide. The reaction may occur within a few minutes after eating in fast cases, and may not appear until 2-3 days in slow cases. Most of them occur between 10 minutes and 2 hours after eating.

A while ago, the neighbor downstairs gave his 7-month-old baby freshly squeezed orange juice for the first time. After feeding him two small spoons, within three or four minutes, the corners of the baby's mouth turned red, and he kept scratching his face. He was full of tears. He was sent to the emergency department and told that he had a citrus allergy. This type of allergy is a typical immediate type of allergy, mostly IgE-mediated immune reactions. The immune system responds quickly and symptoms are straightforward. Parents can usually detect it immediately and rarely delay treatment.

But don’t think that all allergies are so “direct”. Those that are difficult to identify are delayed-onset. My best friend's child ate prawns for the first time when she was over 1 year old. She was bouncing around without any problems that day. However, the next night she started to develop hives and vomiting and diarrhea. At first, the whole family thought she had eaten cold watermelon to make her stomach upset. After a day of searching for allergens, they realized she was allergic to shrimp. This type of non-IgE-mediated allergic reaction has a slow onset, and the symptoms are very similar to common problems such as gastroenteritis and heat rash, making it particularly easy to be misdiagnosed.

Speaking of which, there is no completely unified "latest allergy onset time" in the academic community. The mainstream view is that delayed allergy basically does not exceed 72 hours. However, there are also very few clinical cases that show that in some babies with innate immune sensitivity, allergic reactions will not appear until 4-5 days after eating. However, this proportion is extremely low, less than 5% of the total allergic cases, and most of them are concentrated within 72 hours.

Many novice parents have asked me if adding new complementary foods can be done after two hours of observation and then they will be relieved. In fact, it is not true, especially for highly allergenic foods such as egg whites, nuts, seafood, and mangoes. It is best to observe them for 3 days in a row. Only add one kind at a time, from small to large amounts. If you are allergic, you can quickly find the source and avoid guessing. If the baby has allergies, or if the parents have a history of allergies such as allergic rhinitis or asthma, you should pay more attention when adding new foods and don't be too troublesome. After all, if the baby suffers once, the whole family will have to endure it for several days.