sentences:  The severity of clinical signs and their speed of onset is determined by the dose of alloantibodies taken in by the foal and their potency.Alloantibodies against the Aa blood group are especially potent, and usually produce more severe signs than other alloantibodies when an equivalent dose is absorbed.Mares with multiple exposures to a blood group antigen also produce a greater amount of alloantibodies and therefore the foal receives a larger dose.