Monocytes (Monos)

Monocytes (Monos)

What are monocytes?

Monocytes are the largest types of white blood cells (WBC). They do not contain granules and have a bean-like shape. Monocytes make up from 2 to 9% of the total number of white blood cells (leukocytes). The main work of monocytes is to ensure the immune defense of the body.

All blood cells originate from common parent cells that are bone marrow stem cells (myelopoiesis). Monocytes are produced in the bone marrow which they leave entering the bloodstream being not fully mature cells. These immature cells have the greatest phagocytic ability (they ingest harmful foreign particles, bacteria, and dead or dying cells). Monocytes circulate in the bloodstream for a few days and then migrate into the tissues where they turn into macrophages. Macrophages are found in almost all body tissues. Macrophages are the main “cleaners” of the human body, since their job is to absorb antigens and process them so that they can be recognized by lymphocytes as foreign substances. Most mature monocytic macrophages are located in the liver (56.4%), in the lungs (14.9%), spleen (15%), and peritoneal cavity (7.6%).


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Monocytes are the main cells of the immune system that have marked phagocytic abilities. They can absorb both relatively large elements and a large number of small ones, and, as a rule, do not die after that.

Macrophages are much larger than other forms of WBCs. Macrophages have a longer lifespan and are able to function in an acidic environment, which distinguishes them from eosinophils and neutrophils, which absorb only small elements and then die immediately.

Monocytes absorb microbes, and thereby clean the inflamed area and prepare this area for recovery (regeneration). Monocytes are able to create a kind of protection near large substances that cannot be destroyed. Thus, they protect against viral damage, bacteria, fungi and protozoal infections.

Monocytes are important types of cells in the innate immune system. Scientific evidence suggests that monocytes not only play a crucial role in our innate immune system protecting the body from microbial pathogens but can also contribute to the development of diseases such as liver fibrosis, atherosclerosis, multiple sclerosis, and tumor metastasis [1].

Monocytes in Pregnancy

A healthy pregnancy is associated with the activation and maturation of monocytes. Monocytes influence other cells to form an immune response during pregnancy. Poor placentation leads to the production of more different factors from the affected placenta. This can cause further activation and maturation of monocytes, which leads to a generalized inflammatory response that is typical of preeclampsia and, consequently, to hypertension and proteinuria (protein in the urine). It is believed that pregnancy-induced activation of monocytes is necessary to compensate for changes in adaptive immune responses during pregnancy [2].

Monocytes in Children

The body's defense system consists of innate and adaptive (acquired) immune systems. The innate immune system is the first line of defense against pathogens and consists of physical barriers (skin, epithelium, saliva, etc.), as well as immunological barriers in the form of various immune cells (monocytes, macrophages, neutrophils, etc.) [3]. Monocyte counts in children are usually higher than in adults.

Units of Measure

The absolute number of monocytes in the blood can be expressed in international units:

  • 109 cells/liter
  • G/L – Giga per Liter. (Giga = 1 billion)

Conventional units:

  • thousand per cubic millimeter (1000/mm3, 103/mm3)
  • thousand per microliter (1000/µL, 103/µL)
  • K/mm3 (thousand cells per cubic millimeter)
  • K/µL (thousand cells per microliter)
  • cells/mm3
  • cells/µL

Conversion factors:

Cubic millimeter (mm3) = microliter (mcL, µL)

109 cells/Liter = G/L = 103/mm3 = 103/µL = K/mm3 = K/µL

Normal Range

The normal level of monocytes in the blood of an adult is about 2 to 8% (relative monocyte count). The absolute number (absolute monocyte count) is between 100 and 700 cells/mm3 (0.1-0.7 x 109 cells/L) [4].

Monocytes Normal Range in Pregnancy

The number of monocytes in the blood during pregnancy can be slightly increased, especially in the third trimester:

  • First trimester: 0.1-1.1 x 109 /L
  • Second trimester: 0.1-1.1 x 109 /L
  • Third trimester: 0.1-1.4 x 109 /L

Monocytes Normal Range in Children

The number of monocytes in newborns is significantly higher compared to adults. Monocyte levels gradually increase during the first 2 weeks of life.

About 3 to 8% is considered the normal relative indicator of monocytes in children, the absolute values vary slightly depending on the age of the child:

AgeReference Interval
2-4 weeks0.1-1.7
2 months – 6 years0.2-1.2
6-12 years0.2-1.0
12-18 years0.2-0.8

High Monocytes

An increase in the number of monocytes in the blood that exceeds the upper limit of the reference range is called monocytosis. Monocytosis most often occurs before and after chronic inflammation or infection. However, other conditions, such as heart disease, depression, stress, diabetes, and obesity may be associated with monocytosis. Monocytosis is not a disease, but a condition of blood cells, so monocytosis does not cause symptoms. However, symptoms may occur due to a condition or disease that caused a change in the number of monocytes.

A relative increase in monocytes in a WBC differential is called relative monocytosis (MON%, MO%). An increased percentage of monocytes usually is due to a decrease in other white blood cells (neutrophils, lymphocytes, eosinophils, basophils). In this case, the total WBC number may be normal. Usually this can be observed after recent illnesses. Sometimes the increase can be permanent and is an individual feature of a person.

An absolute monocytosis (MON#, MO#) – an absolute increase in monocytes – occurs when the upper limit of the reference range of monocytes increases on the background of an increase in other white blood cells.

Monocytosis is usually caused by the following conditions [5]:

  • Bacterial infections (tuberculosis, subacute bacterial endocarditis, brucellosis).
  • Other infections (syphilis, viral infections (e.g., infectious mononucleosis), many protozoal and rickettsial infections).
  • Malignancies (chronic myelomonocytic leukemia, monocytic leukemia, Hodgkin's disease, myeloproliferative disorders).
  • Recovery phase after neutropenia or an acute infection.
  • Autoimmune diseases (systemic lupus erythematosus, rheumatoid arthritis, ulcerative colitis, inflammatory bowel disease).
  • Other causes (sarcoidosis, accumulation diseases).

Low Monocytes

A decrease in the proportion of monocytes in the blood is called monocytopenia.

Monocytopenia alone does not cause symptoms. The symptoms are usually associated with an underlying disorder.

A low number of monocytes in the blood can be caused by anything, which reduces the total number of white blood cells, for example, bloodstream infection, chemotherapy, or bone marrow disease. In patients with rheumatoid arthritis, systemic lupus erythematosus and AIDS, a decrease in the number of monocytes in the blood has also been reported.

Severe aortic stenosis is associated with a decrease in the total number of monocytes [6].

Monocytopenia is a characteristic feature of hairy cell leukemia and is considered a diagnostic sign of this disease.

Complete Blood Count (CBC) Interpretation