There is a key difference between an intolerance and an allergy!
An allergy is an reaction of your immunesysteem to the food. It is detectable in blood.
An intolerance is for example when you drink a lot of milk and your intestines can't digest it proberly (missing lactase or other cause). Then you get diarrea. If you have a bad intolerance you will get it with a small amount of milk. It is NOT detectable in blood.
You can also have intolerances for other products.