The nose is the classic target of allergies. Running, sneezing, itching, stuffiness, and post-nasal drip are the all common symptoms of someone in the throes of allergy season. When histamine is released, it goes to work quickly, opening up vessels. Tissues swell, mucus production increases, and itchiness ensues as nerve cells react to the histamine. Other chemical mediators that work alongside histamine can also contribute to stuffy nose.