There are many different types of allergic reactions. I've found numerous ways that they can be classified, so I will just give you the basics.
- Immediate-reaction allergy - This is an IgE mediated, anaphylactic reaction. Often referred to as "true" allergies. Reactions can be severe and life-threatening.
- Delayed-reaction allergy - These are mediated by antibodies other than IgE (IgG, IgM, IgA, etc.) Most food intolerances fall into this category. Reactions often show up from several hours to several days after exposure to a substance, and cause an inflammatory response in the body which can show up in a variety of ways- digestive issues, skin issues, etc.
Other things that could cause allergy-type symptoms:
- Food chemical sensitivity - This isn't actually an allergy (in the sense that it doesn't involve the immune system), but a reaction to specific naturally-occurring chemicals in foods. Most common sensitivities are to salicylates, amines, and others like sulfates and nitrates. Here's a website with more information. FAILSAFE diet is usually recommended.
- OAS (oral allergy syndrome) - This condition usually causes allergy-type symptoms (itching, burning) in the mouth when eating certain raw foods, such as fruits or nuts. It's most common in people with environmental allergies, as it's the pollen that you're reacting to. Many people can tolerate the foods if they're cooked (fruit) or soaked (nuts).