Antihistamines work by blocking histamine, a substance produced by your cells to prevent infection.  If your body detects a foreign substance, your cells will make histamine, which causes inflammation of blood vessels. Usually this is useful. But when your body mistakes a non-harmful substance, like pollen, for a harmful one, it can cause what we call seasonal allergies.  Antihistamines are commonly known for treating seasonal allergies, but there are several other uses for both over-the-counter antihistamines and those prescribed by a doctor. Before taking antihistamines, understand how antihistamines work and what symptoms they treat.