**1. Introduction**

Granulomatous diseases are a heterogeneous group of the conditions of various etiologies with a variety of clinic syndromes and morphological features, nonuniform sensitivity to therapy, and the existence of granulomas. Granuloma is indicative of chronic inflammation involving cells of the macrophage system and other inflammatory cells. After antigen exposure, the activation of T-lymphocytes, macrophages, and epithelioid histiocytes lead to granuloma formation. Granulomas also contain the extracellular matrix produced by fibroblasts, which provide the boundary and isolation of antigens. Their etiology may classify granulomatous diseases as infectious and noninfectious. However, recent studies demonstrate that pathogenic microorganisms may cause the granuloma formation in diseases previously considered as noninfectious. In some cases, differentiation between infectious and noninfectious processes may be problematic. This chapter aims to highlight the multiple forms of granulomatous diseases, characterize the pathologic features of different infectious and noninfectious granulomatosis, and delineate the diagnostic approach.

The term granuloma comes from the Latin word "*granulum*" which means "grain," and the Greek suffix ""-*oma*" used to refer to its nodular formation. Granuloma is a nodular defined formation. This term designates compact cell aggregates on microscopy; granulomas may consist of histiocytes and/or epithelioid cells, giant multinucleated cells, and other inflammatory cells (lymphocytes, neutrophils, and eosinophils). Epithelioid and giant multinucleated cells are monocytes/macrophages derivatives; the former represents good differentiated secretory cells, while the latter specializes in phagocytosis [1]. Giant multinucleated cells are formed by fusion or incomplete cell division, proven in experimental studies [2]; moreover, foreign-body giant cells are formed earlier, than Langhans cells. Lymphocytes are located mainly at the periphery of a granuloma and are represented by T-cells while B-lymphocytes are scattered outside the granuloma. Depending on the disease, T-lymphocytes are predominantly represented by T-helpers 1 and 2 or cytotoxic T-suppressors.

Granulomatous diseases are the heterogeneous group of the diseases of different etiology with a variety of clinic syndromes and morphological features, nonuniform sensitivity to therapy [3]. This chapter aims to highlight the variety of granulomatous lung diseases, to characterize the key morphological features of various diseases of infectious and noninfectious nature, as well as to delineate the diagnostic approach. First, the granulomatous diseases, with arising granulomas which do not lead to necrosis development, with some exceptions are mentioned.
