Respiratory tract infections are a major cause of morbidity and mortality in adults and children worldwide. Because of its anatomical features, which allow gaseous exchange, the respiratory tract is constantly exposed to the outer environment and to the systemic and pulmonary circulation, which may allow infectious microbes, toxins, allergens, dust, and other antigens to enter the lung. The human host is a perpetual battleground between the body's immune system and invading antigens, whether they are microorganisms, chemicals, or cancer cells. Although a vast amount of literature is accumulating on the subject of immune responses to pathogens, the mechanisms underlying specific immunity to many organisms remain unknown. Paradoxically, while the immune response has evolved to confer protection against invading antigens, much human pathology arises when the immune responses are evoked.