I have had may advanced food allergy tests, and it turns out I am gluten and lactose intolerant (not allergice, but intolerant).
With lactose, I discovered I can manage goat cheese, or diery products based off of it, but not cow milk. In diery products, there is also casein besides lactose. I can manage lactose, but not casein.
And I understood that across mammals, the structure of the proteins, lactose, and casein vary slightly. You may be allergic for some of them or all of them.
Finally, I understood that the higher the fat percentage, the lower the amount of lactose and casein, and the more you should be able to handle it (e.g., butter).