An intolerance to feed means all adverse reactions of the body that can be attributed to the intake of food. The causes for this can be either immunological or non-immunological in origin. In the case of food allergies, the immune system reacts to certain components known as allergens in the feed. Both the allergens and the reactions to them differ from animal to animal. In general, however, larger protein components are the cause of allergic reactions. These proteins can originate from animals, but also from plants or even from yeasts or mites. In all cases, it is important to eliminate from the feed what is not tolerated. Hypoallergenic diets are used for this. These can be based on two different approaches: a) Hydrolysed protein sources: in this process, the proteins are broken down to such an extent that they can no longer trigger an allergic reaction. b) Use of selected protein sources to which the animal does not react. Both animal feeds can also be fed for life if there is no reaction.