[2] The mononuclear phagocyte system is also a somewhat dated concept trying to combine a broad range of cells, and should be used with caution.
The monocyte is formed in the bone marrow and transported by the blood; it migrates into the tissues, where it transforms into a histiocyte or a macrophage.
The half-life of blood monocytes is about 1 day, whereas the life span of tissue macrophages is several months or years.
The mononuclear phagocyte system has an important role in defense against microorganisms, including mycobacteria, fungi, bacteria, protozoa, and viruses.
The various cell types of the mononuclear phagocyte system are all part of the myeloid lineage from the CFU-GEMM (precursor of granulocytes, erythrocytes, monocytes and megakaryocytes).