Kurloff cells (also known as Foà-Kurloff cells,) were described as mononuclear cells in the peripheral blood and organs of the guinea pig, capybara, paca, agouti and cavie. The Kurloff cell contains a characteristic proteoglycan-containing inclusion body. In the guinea pig, Kurloff cells are more numerous in the adult female than the adult male. A marked increase in the number of circulating Kurloff cells is present in the peripheral blood during pregnancy and after estrogen treatment in male and female animals. A relatively smaller number of cells take place in immature, non-pregnant, and non-estrogen-treated animals. The exact function of Kurloff cells remains unknown, but it has some of the characteristics of both monocytes and lymphocytes. In guinea-pigs, it has been proposed that Kurloff cells mainly involve in the function of the immune system, such as acting as a natural killer cell and preventing damage to the trophoblast by maternal defensive cells. Also, Kurloff cells present antibody-dependent cytotoxic activity in vitro. Structure The structure of Kurloff cell was identified using light microscopy and periodic-acid Schiff staining. The Kurloff cell has an egg shape and the axis of the cell varies from 8 to 12 µm in length, and 10-25 µm in diameter. The inclusion body, which is round in shape and 1-8 µm in diameter, occupies most of the cell's cytoplasm. Also, the inclusion body resembles a lymphocyte. The nucleus of Kurloff cell is sickle-shaped and is pushed toward the periphery of the cell by the inclusion body. In thymus and spleen, Kurloff cells are categorized depending on the size and the number of inclusion bodies within cells. The cells with small and medium-sized inclusions are present when the density of thymocytes is low. On the other hand, cells with large inclusion bodies were first observed among low-density cells. Over time, they are found where the density of thymocytes is high. Tissue distribution In pregnant and estrogen-treated male and female guinea pigs, clumps of Kurloff cells containing inclusion bodies of different sizes are present in large numbers in stromal tissue of the thymus and bone marrows, and the pulp cords of the spleen. In the thymus, clumps of Kurloff cells are occasionally seen at the cortico-medullary junction, scattered single cells are seen in the cortex, and numerous Kurloff cells are spotted in lymphatics and venules. In the spleen, Kurloff cells are absent in the lymphoid tissue of the white pulp, whereas there