Body processes

Mononuclear phagocyte system – function, role and diseases

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 cells are able to absorb germs, cellular degradation products and foreign particles, render them harmless and transport them away. The system also includes progenitor cells that only develop into cells capable of phagocytosis after appropriate stimulation.

What is the Mononuclear Phagocyte System?

The mononuclear phagocyte system, or MPS for short , includes all body cells that are capable of phagocytosis, i.e. all cells that are able to absorb and kill pathogenic germs in the form of bacteria or viruses and thus render them harmless, as well as degradation particles or absorb foreign particles and transport them away.

Progenitor cells of the mononuclear cells capable of phagocytosis are also assigned to the MPS. Specifically, MPS includes a variety of specialized macrophages that have adapted to the tissue in which they have lodged as quiescent macrophages.

It is somewhat controversial whether microglia, which are capable of phagocytosis in the nervous system , can be counted as MPS, because it is not sufficiently clear whether the microglia developed from monocytes or whether they are transformed glial cells . There is agreement to classify the up to 100 µm large multinucleated osteoclasts in the MPS. The task of the osteoclasts, which are formed from the fusion of up to 25 progenitor cells from the bone marrow and therefore have multiple nuclei, is to break down and transport away bone substance.

The MPS, which was defined in the 1970s, contrasts with the reticulohistiocytic system (RHS), which was developed in the 1920s and is a little broader and, in addition to the phagocytic cells, also includes cells of the reticular connective tissue .

function & task

The most important tasks of the mononuclear phagocyte system are primarily the absorption and control of invading pathogens, the absorption and removal of endogenous waste particles from dead cells (cell detritus) and the absorption and neutralization of 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 absorb the pathogenic germs or particles – similar to amoebas – and enclose them in an inner cavity, the phagosome. The necessary enzymes for killing and decomposing the germs are available in small sacs, the lysosomes , which empty their contents into the phagosome. A kind of digestion process takes place in the phagosome.

In the case of local foci of infection , which can 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 instrument for immune reactions . The various interleukins are synthesized by the activated phagocytes themselves.

An important task in the interaction of phagocytes and progenitor cells with each other for a systemic immune response to viral infections is their ability to act as antigen-presenting cells. Cells containing phagocytosed pathogens present specific peptide fragments (antigen) of the dissected germs on their surface, which are recognized by T helper cells, which initiate the production of specific antibodies .

In the event of a serious virus infection, specialized macrophages in the spleen take over the seemingly absurd replication of the viruses enclosed in their phagosomes in order to be able to produce sufficient amounts of antibodies more quickly. The specialized cells that replicate the dangerous viruses are densely surrounded by macrophages so that any escaping virus can be immediately intercepted for safety reasons. The cells belonging to the mononuclear phagocyte system are also responsible for checking all cells for any degeneration that indicates cancer . As soon as the immune system recognizes cancer cells, macrophages are activated to phagocytose and degrade the body’s own cells recognized as degenerate.

Diseases & Ailments

On the one hand, illnesses and complaints in connection with the mononuclear phagocyte system can result from a 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 the phagocytes, also lead to comparable symptoms.

Typical complaints and diseases that are triggered by a misguided immune reaction are allergic reactions, which involve an excessive immune reaction to certain harmless particles such as pollen, food components or house dust. The spectrum of allergic reactions is very broad and ranges in symptoms from sneezing and slight skin reactions to anaphylactic shock .

In a similar category of overall system malfunctions fall 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 against articular cartilage build up, so misdirected macrophages 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 abnormal production of monocytes, which are part of 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 against phospholipid-binding proteins lead to the increased formation of thrombi , which can lead to occlusion of vital veins and thus to embolism and stroke . Some of the diseases and conditions associated with APS can be attributed to a genetic predisposition.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.