Lymphocytes in the lung: an often neglected cell

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Abstract
The lung is continuously in contact with inhaled particles, some of which are of microbial origin. This requires adequate defence mechanisms in the form of immune reactions. These can be subdivided into the afferent and efferent limb. Specific immune reactions depend on the interactions between lymphoid and accessory cells. Therefore, the local histotopographic localization of lymphocyte subsets has to be known to understand pulmonary immune reactions. As lymphocytes have often not been mentioned when cells in the respiratory tract have been characterized, their compartmentalization, number and subset composition in the lung are outlined here. Lymphocytes are found in the epithelium and lamina propria of the bronchi with different subset compositions. In some species, like the rabbit, bronchus-associated lymphoid tissue (BALT) is found as follicle-like 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. Nevertheless, certain (probably) microbial stimuli can induce BALT in adult humans. In contrast to many other organs, the lung vascular bed contains large numbers of lymphocytes. Little is known about the adhesion molecules that make this margination possible. In the lung interstitium about 10×109 lymphocytes have been calculated for healthy adults. The most easily accessible pool of lymphocytes in the human lung are those recovered by bronchoalveolar lavage. The vast majority of such lymphocytes express markers typical for “memory lymphocytes”. The intrapulmonary migratory routes of lymphocytes and the integration of the lung in the common mucosal immune system are described. A multicompartmental model for lymphocytes is outlined as a basis for understanding lung diseases such as asthma.