Histiocytic sarcoma is a tumor derived from histiocytes. A histiocytoma is a tumour consisting of histiocytes. They may resemble lymphocytes, histiocytes, and plasma cells. The tissue is filled with lymphocytes, plasma cells, mast cells, histiocytes, and eosinophils. These foci consist of collagen rich connective tissue, fibroblasts, histiocytes and osteoclasts. It replicates intracellularly inside histiocytes and nerve cells and has two forms. The parabasal cells can also differentiate into histiocytes or glandular cells. Lymphocytes, histiocytes, multinucleated giant cells and neutrophils are recruited. Medullary sinuses contain histiocytes (immobile macrophages) and reticular cells. Histiocytes (specifically macrophages) are the cells that define a granuloma. The Kupffer cells of the liver and tissue histiocytes are also part of the MPS. The source for histiocytes is the monocyte/macrophage line. Involvement of the superficial dermis is inconsistent and epidermotropism of the histiocytes is not observed. In CT, they give rise to macrophages (histiocytes). Some sources consider Langerhans cell derivatives to be histiocytes.