I almost have common cold every two month. how can i improve immune system.

Drink plenty of : Water. Exercise daily. Eat healthy and get your sleep. Do all these things well and your overall health should improve

Many times: "Common colds" have the same symptoms of "lack of moisture" or allergies or both. Try a steroid nasal spray--Nasal Cort is over the counter now. And when the heat is on in the house, make sure you have a humidifier in the bedroom, and use Ayr saline nasal or gel mist frequently. Keeps your nose :)