LYMPHOCYTES IN THE LUNG - AN OFTEN NEGLECTED CELL - NUMBERS, CHARACTERIZATION AND COMPARTMENTALIZATION

The lung is continuously in contact with inhaled particles, some of wh ich are of microbial origin. This requires adequate defence mechanisms in the form of immune reactions. These can be subdivided into the aff erent and efferent limb. Specific immune reactions depend on the inter actions between lymphoid and accessory cells. Therefore, the local his totopographic localization of lymphocyte subsets has to be known to un derstand pulmonary immune reactions. As lymphocytes have often not bee n mentioned when cells in the respiratory tract have been characterize d, their compartmentalization, number and subset composition in the lu ng are outlined here. Lymphocytes are found in the epithelium and lami na propria of the bronchi with different subset compositions. In some species, like the rabbit, bronchus-associated lymphoid tissue (BALT) i s found as folliclelike aggregations with lymphocytes infiltrating the epithelium, which shows specialized epithelial cells. BALT, however, is not a constitutive structure in all species, e.g in humans. Neverth eless, certain (probably) microbial stimuli can induce BALT in adult h umans. In contrast to many other organs, the lung vascular bed contain s large numbers of lymphocytes. Little is known about the adhesion mol ecules that make this margination possible. In the lung interstitium a bout 10x10(9) lymphocytes have been calculated for healthy adults. The most easily accessible pool of lymphocytes in the human lung are thos e recovered by bronchoalveolar lavage. The vast majority of such lymph ocytes express markers typical for ''memory lymphocytes''. The intrapu lmonary migratory routes of lymphocytes and the integration of the lun g in the common mucosal immune system are described. A multicompartmen tal model for lymphocytes is outlined as a basis for understanding lun g diseases such as asthma.