The human lungs are composed of about 300 million small-sized air sacs, called the alveoli. Defense elements are present throughout the respiratory tract from nose to the alveoli. They can be broadly classified as nonspecific or innate and the specific or adaptive defenses. There are two immune mechanisms for dealing with microbes and other foreign antigens in the lungs, including epithelial cells, dendritic cells, PMN granulocytes and lymphocytes. In addition, we have several genetic and acquired factors that may alter the lung defenses leading to disease. Intrinsic defects, cytotoxic infections and smoking may impair the mucociliary transport leading to stagnation of secretions and infections. It is postulated that modulation of the immune processes, either through the respiratory or the digestive tract, may decrease infections or reduce allergies.