My son was the same way with skin contact.  He has gotten only slightly
better.  The allergist believed that if someone ate a milk product and kissed
him he would have a bad reaction because the acid in the persons mouth would
break down the food a little.  If I touch the back of his hand with a drop of
milk, no real reaction, but if someone kisses him in the same place after
eating, whoooa, watch out!

--Sally (Peter 6, multiple food allergic)