Post nasal drip is an unscientific term that refers to the sensation of thick phlegm in the throat, which can become infected. It is annoying because normally the throat is moistened by the nasal secretions and throat mucous glands. This is part of the mucous - nasal cilia system that defends us from disease. When the amount of liquid secreted by the nose and sinus is reduced, and the cilia of the nose and sinus slow down, the fluid thickens and you become aware of its presence.
http://www.ent-consult.com/pnd.html

The glands in your nose and throat continually produce mucus (one to two quarts a day). It moistens and cleans the nasal membranes, humidifies air, traps and clears inhaled foreign matter, and fights infection. Although mucus normally is swallowed unconsciously, the feeling that it is accumulating in the throat or dripping from the back of your nose is called post-nasal drip.
http://www.entnet.org/healthinfo/nose/nasal.cfm

Dadurch, dass der Schleim aus dem Nasenrückenraum ständig in den Rachen fließt, kommt es zu einer sogenannten Schleimstraße.
http://www.naturheilbund.de/images/tipps/pdf/33.pdf

--------------------------------------------------
Note added at 1 hr 16 mins (2005-07-24 20:16:17 GMT)
--------------------------------------------------

Post Nasal Drip:
According to the World Book Medical Encyclopedia, Post Nasal Drip is defined as, \"a condition in which mucus from the back of the nasopharynx drips down the back of the throat. It may be caused by sinusitis or other inflammatory disorders.
http://www.sinuspharmacy.com/postnasaldrip.html