INFILTRATE ("to enter the open spaces of another sub,, stance" < Med L in- "in, filtrum "filter or porous article through which a fluid is poured")
Elements present in tissue in greater quantity than normal. An infiltrate may be cellular or noncellular. When it is cellular, an infiltrate may be composed of inflammatory cells, e.g., typical histiocytes of xanthogranulomas, or neoplastic cells, e.g., atypical histiocytes of atypical fibroxanthoma (malignant fibrous histiocytoma). When it is noncellular, an infiltrate may consist of mucin, amyloid, colloid, or urate. Noncellular infiltrates are referred to as deposits.
An infiltrate made up of a single type of cell is considered to be monomorphous (G.monos "one, single," G.morphe "form," L. -osus "consisting of"). Examples are mast cells in the benign neoplasm urticaria pigmentosa, small lymphocytes in the inflammatory disease polymorphous light eruption, and large lymphocytes in some malignant lymphomas. An infiltrate that is polymorphous, known also as mixed-cell infiltrate, is composed of several types of cells present together. For example, polymorphous infiltrates of neutrophils, eosinophils, Iymphocytes, plasma cells, and histiocytes are seen in plaques of granuloma faciale and erythema elevatum diutinum, and in some plaques of mycosis fungoides and histiocytosis X. By contrast, a monomorphous infiltrate may be constituted entirely of lymphocytes, as in erythema figuratum, or of histiocytes, as in some expressions of sarcoidosis. "Polymorphous" should not be confused with "pleomorphic", which refers to variations in sizes and shapes of cellular structures, particularly nuclei of neoplastic cells.