In the normal adult human lung, lymphoid tissue is inconspicuous, taking the form of intrapulmonary lymph nodes and the so-called bronchus-associated lymphoid tissue (BALT). It is increased in a number of conditions, both benign and malignant, sometimes as localized lesions and sometimes as a diffuse process. The distribution of lymphoid tissue in both diffuse reactive processes, particularly lymphoid interstitial pneumonia (LIP) and multicentric Castleman disease (MCD), and diffuse neoplastic processes, both primary and secondary, is determined to a great extent by the anatomy of the pulmonary lymphatics. These form a superficial plexus in the pleura, draining along interlobular septa to hilar nodes, and a deep efferent plexus around bronchovascular bundles, draining to the hilum and tracheobronchial nodes. Both benign and malignant infiltrates may be associated with other recognizable pathology, such as systemic disease or interstitial lung disease.
Difficulties in separating and classifying primary lymphoproliferative lesions, coupled with the problem of distinguishing between benign and neoplastic lymphoid infiltrates, has left a legacy of confusing terminology, which has now been largely clarified with the benefits of immunohistochemistry and molecular techniques. In particular, most cases previously labelled “pseudolymphoma” are now recognized as low-grade marginal zone lymphoma, and the diagnosis of LIP has been more clearly defined since its original description. Both reactive and malignant lymphoid proliferations may be localized or diffuse with a wide range of differential diagnoses and these are dealt with in detail under individual section headings. Those malignant lymphomas that are primarily node or bone marrow-based and involve the lungs secondarily are discussed separately, together with pulmonary involvement by leukemic processes. Others, such as intravascular lymphoma and lymphomatoid granulomatosis, are not necessarily confined to the lungs at presentation. However, the pulmonary manifestations are an important component of the primary disease process and they are discussed with primary malignant pulmonary lymphomas.
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