There are actually two components of milk that one can be allergi...

There are actually two components of milk that one can be allergic to - the protein (casein) or the sugar (lactose). My daughter was allergic to the protein at birth, and we (pediatrician & I) knew it was the protein not the sugar, b/c mother's milk is super high in lactose, albeit human lactose and not cow but still is lactose. All babies born into the world have lactase to break down lactose in mother's milk. But some of us must have stopped producing lactase. Wanting my kid to be normal and eat Goldfish, mac 'n cheese as other kids, I tested my daughter at 12 months with Lactaid milk. Diarrhea right away, just like with infant formula. Same thing at 18 months, and at two, I tested her with cheese and organic whole milk. Again, diarrhea. HOWEVER, the only thing she can eat is RAW CHEESE. I would test her on raw milk too if it was legal in my state, but it's not. So my conclusion is this: those who cannot tolerate dairy products or Lactaid may be allergic to casein or milk protein as well. From what I've read, cow milk proteins are very hard to digest, not truly meant for human consumption as much as for cows and calves not only drink the milk raw from mama-cow replete with enzymes but they have four stomachs to help them digest the milk, proteins and sugar and all. This is why some of us have better luck with raw milk (even without 4 stomachs) and why many of us eventually have problems with commercially sold milk from the supermarket that's pasteurized even if we didn't always have problems with it.