Tree allergies are actually allergies to the pollen that trees release in order to reproduce, not to bark or leaves. Trees send out the most pollen in the spring, which means that the allergic rhinitis, or hay fever, caused by trees is most problematic in the spring. When your body reacts to tree pollen, it's the result of you inhaling microscopic substances that your immune system has flagged as harmful. Even though pollen can't really hurt you, your body tries to neutralize the invading allergen. It releases antibodies, which in turn trigger chemicals. The chemical histamine is the one mainly responsible for allergic symptoms. These symptoms include sneezing, itchy and watery eyes, coughing, runny nose and painful sinuses. While the symptoms can be rather annoying, they're your body's way of trying to help. You sneeze in order to get the pollen out of your system, and your nose gets inflamed in order to block more pollen from getting in.