Dear Fiona: The others have explained this in much better detail than I can but I wanted to add something that I heard at a conference about breastfeeding and allergies. The speaker told a story of a child who experienced anaphalactic shock. The mother was aware of the child's extreme sensitivity to cow's milk. She couldn't imagine how the baby got the milk. They finally tracked it down to the diaper rash cream: D______. The wierd thing was that the tube did not list casein as an ingredient. The outside box, that most people throw out, did list casein as an ingredient. Thank goodness this type of sensitivity is very uncommon. Ginny Jones, BSN, IBCLC Florence, SC