Histiocytes: Macrophages found in the TISSUES, as opposed to those found in the blood (MONOCYTES) or serous cavities (SEROUS MEMBRANE).Sea-Blue Histiocyte Syndrome: A congenital disease caused by an inborn error involving APOLIPOPROTEINS E leading to abnormal LIPID METABOLISM and the accumulation of GLYCOSPHINGOLIPIDS, particularly SPHINGOMYELINS in the HISTIOCYTES. This disorder is characterized by SPLENOMEGALY and the sea-blue histiocytes in the spleen and bone marrow after May Grunwald staining.Histiocytosis: General term for the abnormal appearance of histiocytes in the blood. Based on the pathological features of the cells involved rather than on clinical findings, the histiocytic diseases are subdivided into three groups: HISTIOCYTOSIS, LANGERHANS CELL; HISTIOCYTOSIS, NON-LANGERHANS-CELL; and HISTIOCYTIC DISORDERS, MALIGNANT.Histiocytosis, Non-Langerhans-Cell: Group of disorders which feature accumulations of active HISTIOCYTES and LYMPHOCYTES, but where the histiocytes are not LANGERHANS CELLS. The group includes HEMOPHAGOCYTIC LYMPHOHISTIOCYTOSIS; SINUS HISTIOCYTOSIS; xanthogranuloma; reticulohistiocytoma; JUVENILE XANTHOGRANULOMA; xanthoma disseminatum; as well as the lipid storage diseases (SEA-BLUE HISTIOCYTE SYNDROME; and NIEMANN-PICK DISEASES).Histiocytosis, Sinus: Benign, non-Langerhans-cell, histiocytic proliferative disorder that primarily affects the lymph nodes. It is often referred to as sinus histiocytosis with massive lymphadenopathy.Histiocytic Sarcoma: Malignant neoplasms composed of MACROPHAGES or DENDRITIC CELLS. Most histiocytic sarcomas present as localized tumor masses without a leukemic phase. Though the biological behavior of these neoplasms resemble lymphomas, their cell lineage is histiocytic not lymphoid.Histiocytosis, Langerhans-Cell: A group of disorders resulting from the abnormal proliferation of and tissue infiltration by LANGERHANS CELLS which can be detected by their characteristic Birbeck granules (X bodies), or by monoclonal antibody staining for their surface CD1 ANTIGENS. Langerhans-cell granulomatosis can involve a single organ, or can be a systemic disorder.Histiocytic Disorders, Malignant: Distinctive neoplastic disorders of histiocytes. Included are malignant neoplasms of MACROPHAGES and DENDRITIC CELLS.Erdheim-Chester Disease: A rare form of non-Langerhans-cell histiocytosis (HISTIOCYTOSIS, NON-LANGERHANS-CELL) with onset in middle age. The systemic disease is characterized by infiltration of lipid-laden macrophages, multinucleated giant cells, an inflammatory infiltrate of lymphocytes and histiocytes in the bone marrow, and a generalized sclerosis of the long bones.Lymphatic Diseases: Diseases of LYMPH; LYMPH NODES; or LYMPHATIC VESSELS.Granuloma, Plasma Cell: A slow-growing benign pseudotumor in which plasma cells greatly outnumber the inflammatory cells.Histiocytoma, Benign Fibrous: A benign tumor composed, wholly or in part, of cells with the morphologic characteristics of HISTIOCYTES and with various fibroblastic components. Fibrous histiocytomas can occur anywhere in the body. When they occur in the skin, they are called dermatofibromas or sclerosing hemangiomas. (From DeVita Jr et al., Cancer: Principles & Practice of Oncology, 5th ed, p1747)Granuloma: A relatively small nodular inflammatory lesion containing grouped mononuclear phagocytes, caused by infectious and noninfectious agents.Lymph Nodes: They are oval or bean shaped bodies (1 - 30 mm in diameter) located along the lymphatic system.Panniculitis: General term for inflammation of adipose tissue, usually of the skin, characterized by reddened subcutaneous nodules.Epithelioid Cells: Characteristic cells of granulomatous hypersensitivity. They appear as large, flattened cells with increased endoplasmic reticulum. They are believed to be activated macrophages that have differentiated as a result of prolonged antigenic stimulation. Further differentiation or fusion of epithelioid cells is thought to produce multinucleated giant cells (GIANT CELLS).Lymphohistiocytosis, Hemophagocytic: A group of related disorders characterized by LYMPHOCYTOSIS; HISTIOCYTOSIS; and hemophagocytosis. The two major forms are familial and reactive.Malacoplakia: The formation of soft patches on the mucous membrane of a hollow organ, such as the urogenital tract or digestive tract.Xanthomatosis: A condition marked by the development of widespread xanthomas, yellow tumor-like structures filled with lipid deposits. Xanthomas can be found in a variety of tissues including the SKIN; TENDONS; joints of KNEES and ELBOWS. Xanthomatosis is associated with disturbance of LIPID METABOLISM and formation of FOAM CELLS.Mycobacterium scrofulaceum: A non-tuberculous mycobacterium causing cervical lymphadenitis in children. It very rarely causes pulmonary disease, and is believed to be non-pathogenic in animals.Lymphadenitis: Inflammation of the lymph nodes.Histiocytic Necrotizing Lymphadenitis: Development of lesions in the lymph node characterized by infiltration of the cortex or paracortex by large collections of proliferating histiocytes and complete or, more often, incomplete necrosis of lymphoid tissue.Biopsy: Removal and pathologic examination of specimens in the form of small pieces of tissue from the living body.Immunoenzyme Techniques: Immunologic techniques based on the use of: (1) enzyme-antibody conjugates; (2) enzyme-antigen conjugates; (3) antienzyme antibody followed by its homologous enzyme; or (4) enzyme-antienzyme complexes. These are used histologically for visualizing or labeling tissue specimens.Hodgkin Disease: A malignant disease characterized by progressive enlargement of the lymph nodes, spleen, and general lymphoid tissue. In the classical variant, giant usually multinucleate Hodgkin's and REED-STERNBERG CELLS are present; in the nodular lymphocyte predominant variant, lymphocytic and histiocytic cells are seen.Lymphoma: A general term for various neoplastic diseases of the lymphoid tissue.Mononuclear Phagocyte System: Mononuclear cells with pronounced phagocytic ability that are distributed extensively in lymphoid and other organs. It includes MACROPHAGES and their precursors; PHAGOCYTES; KUPFFER CELLS; HISTIOCYTES; DENDRITIC CELLS; LANGERHANS CELLS; and MICROGLIA. The term mononuclear phagocyte system has replaced the former reticuloendothelial system, which also included less active phagocytic cells such as fibroblasts and endothelial cells. (From Illustrated Dictionary of Immunology, 2d ed.)Immunohistochemistry: Histochemical localization of immunoreactive substances using labeled antibodies as reagents.Histocytochemistry: Study of intracellular distribution of chemicals, reaction sites, enzymes, etc., by means of staining reactions, radioactive isotope uptake, selective metal distribution in electron microscopy, or other methods.Rheumatoid Nodule: Subcutaneous nodules seen in 20-30% of rheumatoid arthritis patients. They may arise anywhere on the body, but are most frequently found over the bony prominences. The nodules are characterized histologically by dense areas of fibrinoid necrosis with basophilic streaks and granules, surrounded by a palisade of cells, mainly fibroblasts and histiocytes.