- Also called:
- macrophage system or reticuloendothelial system
- Key People:
- Karl Albert Ludwig Aschoff
- Related Topics:
- immune system
- bone marrow
- macrophage
- reticular cell
mononuclear phagocyte system
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What is the mononuclear phagocyte system?
The mononuclear phagocyte system is a group of cells in the human body that have phagocytosis properties, which allows the cells to engulf and destroy viruses, bacteria, and other foreign substances, while also ingesting abnormal or worn-out body cells.
These phagocytic cells come from precursor cells in bone marrow that develop into monocytes and dendritic cells, which are phagocytic cells that are released into the bloodstream. Some monocytes and dendritic cells stay in blood circulation, but most enter body tissues and develop into larger phagocytic cells known as macrophages.
Macrophages, monocytes, and dendritic cells form a first line of defense against harmful particles that have entered the body by phagocytizing foreign substances. The cells also participate in immune reactions and play a role in destroying worn-out red blood cells and recycling iron.
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Which cells are part of the mononuclear phagocyte system?
The mononuclear phagocyte system is a class of cells distributed throughout the body that perform phagocytosis, which involves engulfing and destroying viruses, bacteria, foreign substances, and abnormal body cells.
Cells of the mononuclear phagocyte system include:
- Monocytes and their precursors in bone marrow
- Macrophages
- Dendritic cells
- Histiocytes
- Kupffer cells
- Microglia
- Alveolar macrophages
Monocytes are produced in the bone marrow and differentiate into macrophages or dendritic cells. Macrophages are found in tissues and ingest microscopic particles, playing a key role in immune responses. Dendritic cells, histiocytes, Kupffer cells, microglia and alveolar macrophages are found in the lymphatic system, subcutaneous tissues, liver, nervous tissue and air spaces of the lungs, respectively.
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What does the mononuclear phagocyte system do in the body?
The mononuclear phagocyte system is a group of cells in the human body that protect against foreign substances and recycle worn-out cells.
These cells originate in bone marrow and differentiate into monocytes and dendritic cells, which enter the bloodstream. In body tissues, monocytes become macrophages that filter and destroy foreign particles. This system defends the body by engulfing harmful particles.
Cells of the mononuclear phagocyte system also participate in immune reactions. Macrophages reveal antigens, stimulating immune reactions directed by lymphocytes, and the production of antibodies stimulates phagocytic activity. In addition, they break down old red blood cells, metabolizing hemoglobin and freeing iron for new red blood cell production.
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How do monocytes become macrophages?
Monocytes, produced in bone marrow, enter the bloodstream as precursors to specialized immune cells. Monocytes differentiate into macrophages within tissues, guided by local cytokines and growth factors like macrophage colony-stimulating factor (M-CSF). This differentiation is essential for engulfing and destroying pathogens, clearing cellular debris, and regulating inflammation. Macrophages, as part of the mononuclear phagocyte system, are found throughout the body and play a key role in initiating immune responses by processing foreign particles for recognition by lymphocytes. Specialized macrophages in tissues include Kupffer cells in the liver and Langerhans cells in the skin.
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What role do macrophages play in fighting infections?
Macrophages are essential in fighting infections as part of the mononuclear phagocyte system. These white blood cells eliminate foreign substances through phagocytosis and initiate immune responses.
Macrophages, which develop from monocytes, ingest and process foreign particles, enabling lymphocytes to recognize and respond to them. They can be fixed in tissues or wander through connective tissues, ingesting cells, infectious agents, and microscopic particles.
Infected tissues trigger an inflammatory response, with macrophages releasing chemicals that cause pain, swelling, redness, and fever. This process increases blood flow and capillary permeability, attracting more phagocytic cells to eliminate pathogens and clear cellular debris.
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How does the mononuclear phagocyte system help remove dead cells and waste from tissues?
The mononuclear phagocyte system (MPS) is a crucial part of the immune system that helps remove dead cells and waste from tissues. Cells of the MPS, such as macrophages, identify, engulf, and destroy bacteria, viruses, minute foreign bodies, and worn-out or abnormal body cells through phagocytosis. This process involves the cell surrounding and ingesting the unwanted material, effectively cleaning the tissues.
The MPS also breaks down old red blood cells, recycling iron for new cells. Found throughout the body, including the lungs, liver, and skin, MPS cells form a first line of defense against harmful particles.
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mononuclear phagocyte system, class of cells that occur in widely separated parts of the human body and that have in common the property of phagocytosis, whereby the cells engulf and destroy bacteria, viruses, and other foreign substances and ingest worn-out or abnormal body cells. German pathologist Karl Albert Ludwig Aschoff introduced the term reticuloendothelial system in 1924, collating the cells based on their phagocytic activity. The later reclassification of phagocytic mononuclear cells, however, resulted in the exclusion of endothelial cells and fibroblasts from the system; hence, Aschoff’s term was replaced in the latter part of the 20th century with the name mononuclear phagocyte system.
Mononuclear phagocytic cells are derived from precursor cells in the bone marrow. These precursors develop into monocytes and dendritic cells, phagocytic cells that are released into the bloodstream. Some monocytes and dendritic cells remain in the general blood circulation, but most of them enter body tissues. In tissues, monocytes develop into much larger phagocytic cells known as macrophages. The great majority of macrophages remain as stationary cells within tissue, where they filter out and destroy foreign particles. Some of them break away, however, and wander through the circulation and within the intercellular spaces.
Cells of the mononuclear phagocyte system differ in appearance and name because of their various locations. For example, dendritic cells are found in many tissues, including the lungs, the skin, and the gastrointestinal tract, as well as throughout the lymphatic system. Histiocytes are found in numerous subcutaneous tissues. Kupffer cells line the sinusoids of the liver. Microglia occur in nervous tissue, and alveolar macrophages are found in the air spaces of the lungs.
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Each phagocytic cell can engulf and destroy microorganisms, cells, and even tiny fragments of foreign objects, such as bits of splinters and suture materials. Several mobile macrophages can surround larger foreign objects and coalesce into a single phagocytic cell. Thus, by their phagocytosis of foreign substances, macrophages, monocytes, and dendritic cells form an important first line of defense against harmful particles that have reached the body’s interior.
Cells of the mononuclear phagocyte system also participate in immune reactions, in which a complex set of events is targeted at a specific foreign substance. Through phagocytosis, macrophages reveal antigens (surface molecules) on foreign substances. Antigens stimulate immune reactions that are directed by white blood cells known as lymphocytes. B lymphocytes (or B cells) synthesize and secrete antibodies with the help of T lymphocytes (or T cells; T cells are also capable of other immunological reactions not involving antibody production). The production of antibodies, in turn, greatly stimulates the phagocytic activity of the cells of the mononuclear phagocyte system.
The mononuclear phagocyte system also plays an important role in the destruction of worn-out red blood cells and the recycling of iron. Specialized macrophages, primarily those residing in the bone marrow, liver, and spleen, break down old red blood cells and metabolize the hemoglobin (the oxygen-carrying pigment of red blood cells), thereby freeing the iron compound heme for the production of new red blood cells.
Disorders associated with the mononuclear phagocyte system include anemia caused by the excessive destruction of red blood cells. There are also malignant tumours related to mononuclear phagocytes that can be either localized or widespread throughout the body; the excessive proliferation of histiocytes, for example, occurs in malignant histiocytosis and monocytic leukemia. Niemann-Pick disease and Gaucher disease are hereditary disorders characterized by abnormal products of lipid metabolism within cells of the mononuclear phagocyte system.
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