Histiocytes are a type of immune cell. They help the body heal after an injury or infection by removing dead cells, blood, micro-organisms (such as bacteria and fungus), and foreign material from the body. Foamy is a term pathologists use to describe histiocytes that are full of waste removed from the surrounding tissue.
Pathologists commonly see large groups of foamy histiocytes in tissue samples after an injury or infection. These cells can be seen when tissue is examined under the microscope using a routine stain called H&E. However, a pathologist can perform a special test called immunohistochemistry to confirm that the cells that they are seeing under the microscope are histiocytes. When this test is performed, these cells will be positive or reactive for CD68 and CD163.