Lymphadenopathies are reactive processes of lymph nodes in response to a variety of exogenous and endogenous stimulants. The etiology and pathogenesis of many lymphadenopathies are still unknown; thus, the category remains heterogeneous and poorly defined. Possible causes of lymphadenopathies include microorganisms not yet identified, autoimmune diseases, immune deficiency and dysregulation, foreign bodies, medical procedures, and tumors. Lymphadenopathies are non-neoplastic processes, so their recognition and differential diagnosis are of great importance in order to rule-out the neoplastic diseases.

Classifying entities of unknown etiologies and overlapping histologic patterns is a tentative and imperfect process. We have chosen the term lymphadenopathies for lymph node lesions in which no infectious agent has been identified, in contrast to lymphadenitides (described in Part III), which are causally related to microorganisms. We have further subclassified lymphadenopathies into Reactive lymphadenopathies (section one), Lymphadenopathies associated with clinical syndromes (section two), Iatrogenic lymphadenopathies (section three), Vascular lymphadenopathies (section four), Foreign body lymphadenopathies (section five), and Lymph node inclusions (section six).