They circulate through the body and enter various organs, where they undergo differentiation into histiocytes, which are part of the mononuclear phagocytic system (MPS).
However, the term histiocyte has been used for multiple purposes in the past, and some cells called "histocytes" do not appear to derive from monocytic-macrophage lines.
These histiocytes are part of the immune system by way of two distinct functions: phagocytosis and antigen presentation.
Macrophages and dendritic cells are derived from common bone marrow precursor cells that have undergone different differentiation (as histiocytes) under the influence of various environmental (tissue location) and growth factors such as GM-CSF, TNF and IL-4.
They also express S-100, and their cytoplasm contains tennis-racket like ultra-structural inclusions called Birbeck granules.