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Epithelioid histiocytes (Epithelioid cells) are activated macrophages resembling epithelial cells. Structurally, they are elongated, with finely granular, pale eosinophilic (pink) cytoplasm, and central, ovoid nuclei (oval or elongate), which are less dense than that of a lymphocyte. They have indistinct shape and often appear to merge into one another, forming aggregates known as giant cells. Epithelioid cells are central in the formation of granulomas, which are associated with many serious diseases.
Epithelioid cells are an essential characteristic of granulomas: without them a histological finding is not a granuloma. A granuloma can be defined as an organized collection of epithelioid macrophages. More broadly, a granuloma may be considered an organized collection of macrophages, including mere collections of giant cells surrounding inert substances like suture material – the so-called "non-immune granulomas." Granuloma formation is associated with pathogens that have learned to evade the host immune system by various means like resisting phagocytosis and killing within the macrophages. Indigestibility of matter by macrophages is a common feature of granulomatous inflammation. Granulomas try to wall off these organisms and prevent their further growth and spread. Historically widespread and destructive diseases such as tuberculosis, leprosy and syphilis are granulomatous conditions. Granuloma formation is also the feature of many more contemporary conditions, like fungal infections, sarcoidosis and Crohn's diseases.
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