When confronted with a classic food allergy your body reacts almost immediately. On the other hand, the reaction to a delayed food allergy (or intolerance to foods) may take days (up to 72 hours) to appear. This difference in speed of reaction is caused by the immune system producing different antibodies in the fight against the different allergies.
In the case of a classic food allergy, also known as allergy type I, the immune system produces IgE antibodies. Someone with a type I allergy to peanuts will probably know immediately. The symptoms will appear within seconds or minutes – swelling, breathing difficulties, rash, itchy skin and even anaphylactic shock. Identification of such an allergy is normally pretty straightforward, with IgE tests used for confirmation of the allergy (traditionally, a scratch or skin-prick test).
More common are delayed food allergies (allergy type III), in which the immune system produces IgG antibodies. These cause a delayed reaction (as long as three days) leading to inflammation which can become chronic.
Because of the delay in symptoms manifesting, it can be extremely difficult to pinpoint which foods causing a type III allergy. If you want to learn about what food can we be intolerant to, click here to read my article on the subject.