Principal cell in granuloma is -

histiocytes are also called epithelioid cells which are modified macrophages. They are elongated cells having a slipper-shaped nucleus. Macrophages and monocytes engulf the antigen and try to destroy it but since the antigen is poorly degradable these cells fail to digest and degrade the antigen and instead undergo morphologic changes to transform into epithelioid cells. ref robbins 9th ed page 98 .