The term mononuclear phagocyte system encompasses all body cells that are capable of phagocytosis and are thus part of the immune system. The cells are capable of taking up disease germs, cellular degradation products, and foreign particles, rendering them harmless, and transporting them away. Progenitor cells, which develop into phagocytosis-capable cells only after appropriate stimulation, are also counted as part of the system.
What is the mononuclear phagocyte system?
The term mononuclear phagocyte system includes all body cells that are capable of phagocytosis and are therefore part of the immune system. The mononuclear phagocyte system, or MPS for short, includes all body cells that are capable of phagocytosis, i.e., all cells that are capable of taking up pathogenic germs in the form of bacteria or viruses, killing them and thus rendering them harmless, as well as taking up degradation particles or foreign particles and transporting them away. Precursor cells of the mononuclear cells capable of phagocytosis are also attributed to the MPS. Specifically, a variety of specialized macrophages that have adapted to the tissue in which they have nested as resting macrophages are counted as part of the MPS. There is some controversy as to whether microglia capable of phagocytosis in the nervous system can be counted as part of the MPS, because it is not sufficiently clear whether microglia evolved from monocytes or are transformed glial cells. There is agreement that the multinucleated osteoclasts, which are up to 100 µm in size, should be included in the MPS. The function of osteoclasts, which form from the fusion of up to 25 bone marrow progenitor cells and therefore have multiple nuclei, is to break down and remove bone substance. The MPS, defined in the 1970s, is contrasted with the reticulohistiocytic system (RHS), developed as early as the 1920s, which is a bit broader and includes cells of the reticular connective tissue in addition to phagocytosing cells.
Function and task
The main tasks of the mononuclear phagocyte system are primarily to ingest and combat invading pathogens, to ingest and remove endogenous waste particles from dead cells (cellular detritus), and to ingest and render harmless foreign particles. In a complex interaction within the MPS, the dormant macrophages in the corresponding tissue are converted into active macrophages by cytokines and messenger substances. They enlarge and take up the pathogenic germs or particles – similar to amoebae – and enclose them in an inner cavity, the phagosome. The enzymes necessary to kill and decompose the germs are available in small vesicles, the lysosomes, which empty their contents into the phagosome. A kind of digestion process takes place in the phagosome. In the case of local infection foci, which may result from injuries, the MPS controls inflammatory reactions and subsequent healing. In this context, the production of various cytokines (interleukins) with pro-inflammatory and also anti-inflammatory effects is an important control tool for immune responses. The various interleukins are synthesized by the activated phagocytes themselves. An important role in the interplay between phagocytes and progenitor cells for a systemic immune response to viral infections is their ability to act as antigen-presenting cells. Cells containing phagocytosed pathogenic germs present specific peptide fragments (antigen) of the disassembled germs on their surface, which are recognized by T helper cells that initiate the production of specific antibodies. In the event of a serious viral infection, specialized macrophages in the spleen take over the replication of the viruses, which at first seem absurd, enclosed in their phagosomes in order to produce antibodies in sufficient quantities more quickly. The specialized cells that replicate the dangerous viruses are tightly surrounded by macrophages so that, for safety reasons, any escaped virus can be immediately intercepted. The cells belonging to the mononuclear phagocyte system are also responsible for checking all cells for any degenerations indicative of cancer. As soon as the immune system recognizes cancer cells, macrophages are activated to phagocytose and break down the body’s own cells that have been recognized as degenerated.
Diseases and ailments
Diseases and disorders associated with the mononuclear phagocyte system can result, on the one hand, from functional impairment of the cells belonging to the system itself. On the other hand, malfunctions or failures within the stimulating part of the immune system, i.e., due to too weak or too strong stimulation and activation of phagocytes, also lead to comparable symptoms. Typical complaints and diseases triggered by a misdirected immune reaction are allergic reactions involving an excessive immune response to certain harmless particles such as pollen, food components or house dust. The spectrum of allergic reactions is very broad, ranging in symptoms from sneezing and mild skin reactions to anaphylactic shock. Falling into a similar category of whole-system dysfunction are the multitude of well-known autoimmune diseases such as multiple sclerosis, Hashimoto’s, rheumatoid arthritis, and many others. In the case of rheumatoid arthritis, antibodies form against articular cartilage, causing misdirected macrophages to attack the articular cartilage, which gradually leads to the sometimes severe and painful symptoms and discomfort. All autoimmune diseases have in common that the phagocytes belonging to the MPS classify endogenous cells of a certain organ as foreign and fight them with corresponding serious effects. Diseases that lead to impaired production of monocytes belonging to the MPS are certain forms of leukemia, a cancer of the bone marrow. An example of a disease caused by misdirected antibody production is antiphospholipid syndrome (APS). Antibodies to phospholipid-binding proteins lead to increased formation of thrombi, which can lead to occlusion of vital arteries, resulting in embolisms and strokes. Some of the diseases and conditions associated with GSP can be attributed to a genetic predisposition.
