The defense of the lung: studies of the role of cell-mediated immunity.

The defense of the lung against infections, toxins and allergens is accomplished by an excretory transport mechanism and by the interaction of cellular and humoral defense systems. Pulmonary alveolar macrophages represent a common effector pathway for both nonspecific cellular phagocytic defenses and for specifically triggered cell-mediated immunity, via T lymphocytes. Nonspecific activation of macrophages is induced by toxic substances. Studies of the immunocytologic system indicate partial compartmentalization and "local" immunity for both cellular and humoral systems. Further studies on pulmonary cell-mediated immunity have characterized an amplification mechanism by which antigen-induced stimulation of T lymphocytes leads to recruitment of nonsensitive lymphocytes through the production of a low-molecular weight "transfer factor." Other lymphocyte-produced mediators (lymphokines) act to attract, aggregate and accumulate macrophages in areas of inflammation. In addition, macrophages are "activated" and show enhanced microbicidal capabilities as well as enhanced resistance to the cytotoxic effects of certain ingested microorganisms. It is postulated that cellular (nonspecific) and cell-mediated (specific) immune defenses play important roles in protection against several categories of microorganisms in a hierarchy of virulence.