The processes for making various dairy products such as sour cream, yogurt, cheeses, etc change the chemical make up of the product. If you are allergic to milk, then you'll most likely only have problems with derivatives that have a more similar make up than a dissimilar one.
Other similar situations:
1) People with OAS (Oral Allergy Syndrome) have problems with raw fruits and veggies but none once they are cooked
2) People with an allergy to sulfa drugs may have problems with other drugs that have a similar chemical make up but are not technically sulfa drugs.
