What is a feed allergy actually?

In recent years there has been an increase in the number of so-called "feed allergies". Generally speaking, one notices the presence of such a feed allergy on the skin, with symptoms such as itchiness, increased licking, increased scaling or pustules or even eczema-like changes on the ears, with inflammation, itchiness, etc. in the intestinal tract, with chronic diarrhoea, chronic inflammation of the digestive tract etc.. As a general principle one should distinguish between two kinds of feed allergies: the primary and the secondary.