Benign Hematopoietic Disorders of the Lung
The lung is an organ constantly exposed to antigens and pathogens that frequently induce local inflammation. This inflammation produces hyperplasia of the bronchial-associated lymphoid tissue (BALT) that can result in exuberant reactive lymphoid proliferations either as interstitial infiltrates or mass-forming lesions (Koss, Semin Diagn Pathol 12:158-171, 1995). Similarly, immune dysregulation or autoimmune disorders can also induce hyperplastic proliferation of the BALT. Reactive pulmonary lymphoid hyperplasia includes follicular bronchiolitis and nodular lymphoid hyperplasia, whereas diffuse lymphoid hyperplasia (lymphoid interstitial pneumonia) is a lesion that some authors believe forms part of the spectrum of lymphoproliferative disorders that may progress to pulmonary marginal zone lymphoma of the mucosa-associated lymphoid tissue (pulmonary MALT lymphoma). Although distinction of these lesions is straightforward in a surgical resection, the diagnosis may be challenging in small biopsies or specimens with crush artifact. In this situation, the use of ancillary studies is helpful to arrive to the diagnosis. IgG4-related lung disease is also discussed in this chapter.
