Flax seed is high in beneficial fats, especially alpha-linoleic acid (ALA) which likely has some anti-allergic properties. I have recommended it for several years for horses with a variety of skin allergies. It seems to make a difference to these horses when fed in adequate quantity (16 oz/ day for a 1000 lb horse).
Flax seed oils: The average ALA (Alpha Linolenic Acid) content of flax seed is 22.8% (NRC 2000). The equivalent amount of oil to be fed is 8oz.
Flax is also a great source of calories and is an excellent coat supplement. It is sometimes overfed to insulin resistant laminitic horses, causing weight gain and worsening of signs.
It should be ground to be utilized properly. In that case it is fed as fresh, ground seed but must be ground right before feeding, or it loses many of its properties. Ideally, the fresh seed is kept refrigerated. There are a variety of stabilized commercial preparations now available as well that do not require such intensive preparation.