Food related rashes of the type you describe, which sounds like urticaria or hives, usually start within minutes, or at most an hour or so, of eating the food concerned. At present, there is no method of desensitising patients with such food allergies, but they do sometimes improve with the passage of time. Avoidance, or if this is difficult, taking a protective dose of an antihistamine, are the only options. In some cases allergic reactions are lessened if the food is cooked, but this obviously isn't an option for all foods.