Vitamin A is important for good eye sight, especially at low light levels (i.e. at night) and is also important in maintaining the integrity of the mucosal cells which are the first defense against invading organisms. Thus, night blindness and susceptibility to infections are signs of a vitamin A deficiency. Reproductive disorders, such as increased frequency of abortions, retained placentas, and birth of weakened, blind or dead calves may also be caused by a vitamin A deficiency.