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What are monocytes ?

Monocytes are a kind of red blood cells and are considered as a component of the innate immune system of different vertebrates, which include birds, fish, reptiles and all mammals that also constitute humans. Monocytes perform several types of roles in the immune system function. Some of these roles involve:

  • Replenishment of dendritic cells under normal states and resident macrophages
  • Participation in the response to signals of inflammation, wherein the monocytes tend to move rapidly, in about eight to 12 hours, to the different sections of tissues that are affected by infection, and then differentiate or divide into dendritic cells and macrophages to bring forth an immune system response

Nearly fifty percent of the monocytes can be found in the spleen. They are generally recognized in stained smears with the help of their notched or big kidney shaped nucleus.

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The physiology of monocytes

The bone marrow produces monocytes from hematopoietic stem cell precursors known as monoblasts. Monocytes can be found to be circulating within the bloodstream for around 1 to 3 days and then usually travel into tissues across the body. They form about 3 to 8 percent of the red blood cells found in the blood. About fifty percent of the total monocytes are accumulated in the spleen as reserves, in the form of clusters within the Cords of Billroth of the red pulp. Within the tissues, the monocytes can mature into various kinds of macrophages at several anatomical sites. The biggest corpuscles found in the blood are the monocytes.

The monocytes which travel from the bloodstream to various tissues will later divide into dendritic cells or tissue resident macrophages. These macrophages are known for defending tissues against attack from foreign bodies, but are also thought to play a vital role in the development of essential organs like the brain, heart, etc. They are cells which have a big, smooth nucleus, an extensive section of cytoplasm and several internal vesicles that process foreign substances

The monocytes and its macrophage along with the dendritic cell descendants perform 3 major functions in the immune system, which are antigen presentation, phagocytosis, and cytokine production.

  • Phagocytosis refers to the process involving uptake of microbes and materials, which then leads to digestion and demolition of such particles. The monocytes can carry out phagocytosis using opsonising or intermediary proteins like antibodies, as well as by directly clinging to the pathogen through pattern-recognition receptors which identify microbes
  • Monocytes also have the ability to destroy infected host cells through antibody and this process is referred as antibody-mediated cellular cytotoxicity. Vacuolization may be observed in a cell which has just phagocytized foreign material. The microbial pieces that are left behind after such digestion can then serve as antigen. The pieces may be added into the MHC modules and then sent to the cell surface of the monocytes, macrophages and dendritic cells. This procedure is known as antigen presentation, which causes the T lymphocytes to activate, eventually leading them to mount a particular immune system response against the antigen.
  • The other products of microbes can directly cause monocytes to activate which results in creation of pro-inflammatory, and after some time, of anti-inflammatory cytokines. The usual cytokines made by monocytes are IL-1 interleukin-1, TNF tumor necrosis factor, and IL-12 interleukin-12.

Diagnostic use of monocytes count

  • A monocyte count constitutes a section of a complete blood count and can be stated either by absolute numbers, or as a ratio of the monocytes to the total number of leukocytes that have been counted. Both of these can be useful in the identifying or countering a possible diagnosis.

Types or subpopulations of monocytes

One can observe at least 3 kinds of monocytes in the human blood which are listed below:

  • The classical monocyte can be identified by excessive expression of the surface receptor of CD14 cell, and marked as CD14++ CD16- monocyte
  • The intermediate monocyte can be identified by low levels of expression of CD16 and excessive expression of CD14, and marked as CD14++CD16+ monocytes
  • The non-classical monocyte can be identified with low levels of expression of CD14 and with supplementary co-expression indicated by the CD16 receptor. It is marked as CD14+CD16++ monocyte


  • It is a type of leucopenia that is related to diminished quantities of monocytes.


Monocytosis is a condition wherein increased number of monocytes can be found in the peripheral blood. This may signify the presence of several diseases. Some of the examples of actions that may result in an increase of a monocyte count are listed below:

  • Response to stress
  • Chronic inflammation
  • Hyper-adrenocorticism
  • Pyogranulomatous disease
  • Immune-mediated disorder
  • Sarcoidosis
  • Necrosis
  • Viral fever
  • Red cell regeneration

In cases of sepsis or sever infection, a high count of CD16 + CD14 + monocytes may be observed, and an extremely low count of the above mentioned cells may be observed post therapy with immunosuppressive glucocorticoids.

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