The importance of lymphocytes in pulmonary health and disease

Lymphoid tissue and lymphocytes are distributed extensively throughout the airways and lung parenchyma and exist in a variety of forms, such as lymph nodes, submucosal lymphoid nodules and aggregates, scattered lymphocytes/plasma cells beneath the lamina propria membrane and free cells on airway surfaces. It is speculative to state precisely how the lymphoid components serve lung host defense or systemic immunity. Yet, anatomically they appear to be strategically located to intercept inhaled antigens and to initiate immune responses. Morever, lymphocytes are part of the effector limb of lung immunity and differentiate to synthesize immunoglobulins (B cells and plasma cells) or participate in cell-mediated reactions (T-cell function). Because of its active lymphoid structures, the lung should be considered as part of the lymphoreticular system. Lymphocyte research in the lung is still at the stage of identifying subpopulations of freely recoverable lymphocytes which can be obtained from normal lung lavage fluids. Increasingly, however, lung lymphocytes are being analyzed for their role in inciting or perpetuating a variety of lung diseases. Whereas cigarette smoking may not be categorized as a disease, it produces some interesting changes in T lymphocytes by diminishing their proliferative response to plant mitogens. Such an alteration in cell function may contribute to lung disease in the future. Several lung diseases which are characterized by hypersensitivity phenomena and a granulomatous response have increased numbers of lymphocytes in lung parenchyma and the airways. Such examples as sarcoidosis and hypersensitivity pneumonitis (extrinsic allergic alveolitis) are discussed as protypes. Lung lymphocytes sampled directly from affected lung tissue are more representative of the pathologic processes and may have different dynamic properties than circulating peripheral blood lymphocytes. Thus, analysis of lymphocytes in lung by relatively noninvasive means may help with diagnosis of disease and improve our understanding of pathophysiology.