Phlegm is a thick, sticky substance produced by the respiratory tract. It is composed of mucus, which is produced by the mucous membranes that line the respiratory tract, and white blood cells, which help to fight infection. Phlegm can be clear, white, yellow, or green. Clear or white phlegm is usually a sign of a mild infection, while yellow or green phlegm is usually a sign of a more serious infection.
There are a number of things that can cause phlegm production, including:
