Pollen allergies, also called hay fever or allergic rhinitis, are the result of your immune system's overreaction to certain airborne substances released by plants. When you breathe them in or they land on your eye or nose membranes, your body reacts. You release antibodies called immunoglobulin E, and they seek out the allergens. Once they're located, the antibodies deliver them to your mast cells, which then proceed to destroy them. In the process, a large amount of chemicals are released into your bloodstream. One of these chemicals, histamine, is responsible for most of your pollen allergy symptoms.
Such symptoms include red, puffy eyelids; inflamed sinuses; sneezing; a runny nose; congestion; post-nasal drip; itchy nose and eyes; and dark circles under your eyes. The idea behind these reactions is that you swell up to stop more allergens from getting into your system, and you sneeze and cough to expel the ones that are already in you.