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Functional Considerations: The Mononuclear Phagocytotic System

The cells that are included in the mononuclear phagocytotic system {MPS) are derived from monocytes and denote a population of antigen-presenting cells involved in the processing of foreign substances. These cells are able to phagocytose avidly vital dyes such as trypan blue and India ink, which makes them visible and easy to identify in the light microscope. The common origin of MPS cells from monocytes serves as the major distinguishing feature of the system as it is currently perceived and is the basis for the system's name. In addition, cells of the MPS display receptors for complement and Fc fragments of immunoglobulins. The various cells of the MPS are listed in Table 5.5.

Most cells of the MPS become fixed in specific tissues and may adopt a variety of morphologic appearances as they differentiate.

The main functions of MPS cells are phagocytosis, secretion (lymphokines), antigen processing, and antigen presentation to other cells of the immune system. Some functionally important phagocytotic cells are not derived from monocytes. For example, microglia are small, stellate cells located primarily along capillaries of the central nervous system, which function as phagocytotic cells. They are generally thought to arise from the mesectoderm of the neural crest and not from monocytes; nevertheless, they are included in the MPS. Similarly, fibroblasts of the subepithelial sheath of the lamina propria of the intestine and uterine endometrium have been shown to differentiate into cells with morphologic, enzymatic, and functional characteristics of connective tissue macrophages.

table 5.5. Cells of the Mononuclear Phagocytotic System

Name of Cell

Macrophage (histiocyte) Perisinusoidal macrophage (Kupffer cell) Alveolar macrophage Macrophage

Pleural and peritoneal macrophage

Osteoclast

Microglia

Langerhans' cell

Fibroblast-derived macrophage

Location

Connective tissue

Liver

Lungs

Spleen, lymph nodes, bone marrow, and thymus

Serous cavities

Bone

Central nervous system Epidermis

Lamina propria of intestine, endometrium of uterus fvh

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