I will cut and paste my response to a request from a few weeks ago:
AND - While many people do not have the lactase (LCT) enzyme needed to properly hydrolyze lactase, fortunately many people indeed, are able, to produce sufficient amounts of lactase predominantly within the villi of their small intestines. So, it is not true that people do not have the ability to digest the lactose in milk.
"You may wish to do some research on the differences between food allergies and food intolerances. Also, check with your pediatrician before diagnosing the issue on your own. Less than eight percent of children under the age of three have a true food allergy. Unfortunately many parents see signs or symptoms of something wrong, automatically jump to the conclusion that there is a food allergy, and then swear off for life all food products that contain the culprit ingredient. That may or may not be necessary.
A food allergy triggers a response in your body that makes your immune system think that the body is under attack by a harmful, foreign substance. As a result, antibodies are produced to combat the substance, causing the immune system to release histamine that can lead to hives, swelling of the mouth/tongue/throat, trouble breathing nausea, anaphylaxis etc. Because the antibodies were produced in response to the food item, the next time the food is ingested (even in a tiny amount) the immune system kicks in and could cause a serious allergic reaction.
A food intolerance can cause many of the same symptoms, including a rash, but the main difference between an allergy and an intolerance is that the allergy, if escalated enough, could be fatal (the swelling of the mouth/tongue/throat can hinder the ability to breathe) and the intolerance doesn't generally involve the immune system and makes a person feel absolutely miserable. People with a food intolerance might be able to eat the food item in small amounts with little to no discomfort and definitely no threat to their life whereas people with a food allergy HAVE to avoid the food item.
Where the food allergy is an immune system response, the food intolerance is a digestive system response. Typically a food intolerance (at her age) is likely due to a lack of a enzyme needed to appropriate break down a food item (this could DEFINITELY be the case for the milk issue; lactose intolerance is the most common food intolerance) or a sensitivity to food additives.
Definitely address these concerns with her pediatrician. In the meantime, your best bet might be to slow down or stop the offending food item until you've had a chance to discuss things with her doctor. Good luck and I hope everything turns out okay!"
If your child had a true food allergy, the reaction would likely be much more severe and potentially life-threatening. Did this happen? If not, it is likely an intolerance.