The Role of Monocytes in the Immune Response

Like all white blood cells, monocytes are made in the bone marrow. Under normal circumstances, the spleen stores a huge reserve of monocytes. When injury or inflammation occurs, the monocytes leave the spleen and head to the peripheral tissues. They differentiate into macrophages or dendritic cells as needed to control the spread of disease or infection.

Macrophages play a defensive role in the immune system. These monocytes are responsible for phagocytosis, the process of engulfing and destroying microbes, though the experts do not completely understand the mechanism they use to kill these invaders. Macrophages also regulate another type of white blood cell — lymphocytes — particularly in response to antigens, to enhance the immune response.

Monocytes may also become dendritic cells, which are found in tissues and display antigens to other cells of the immune system. They serve as sentinels, triggering an immune response. Researchers believe that exploring the ability of the dendritic cells to regulate the immune system could help develop effective treatments for cancer and autoimmune diseases and to prevent transplant rejection.

A test called a complete blood count measures the number of monocytes in the blood, among other things. Doctors use this test to help diagnose symptomatic patients, but it is often included in routine lab work and may reveal an abnormal monocyte count by chance. Certain cancers can cause increased monocytes, as well as autoimmune disorders, blood disorders, and chronic infections. Elevated macrophages appear in organ tissues following sarcoidosis and some infections.

A complete blood count also detects low monocytes, which can indicate a number of problems, including a blood infection or bone marrow disorder like aplastic anemia. Decreased monocytes are also a side effect of chemotherapy and monoMAC, a rare genetic disorder affecting the bone marrow.

Monocytes are an essential part of the immune system, but research suggests they can be a double-edged sword that both helps and hinders healing from some conditions. Some research studies show that monocytes rapidly migrating to a fibrotic liver in an attempt to heal the tissue can increase the severity of the disease. That said, when they differentiate into macrophages, they can prevent additional damage.

Atherosclerosis occurs when cholesterol accumulates on artery walls, reducing the space for blood flow. One study shows that this inflammation of the arteries attracts macrophages that are unable to clear the plaques, thus worsening the problem. This study also showed that, in mice, the progression of atherosclerosis slowed and was not as severe when the infiltrating monocytes and macrophages had a shorter lifespan.

Studies also show that monocytes may play a role in tumor metastasis. Some cells derived from monocytes are known to suppress anti-tumor cells.  One study shows that blocking a major monocyte attractant reduced the spread of the tumor. Inhibiting these factors may [also] prohibit tumor growth. That said, these studies used mice, and more research is needed in human studies.

Studies have also shown that certain monocytes can enter the brain. Experiments on mice indicate the cells can play a role in early central nervous system inflammation. Another area of interest is in the relationship between monocytes and Alzheimer's disease and whether the effects make plaques in the brain better or worse. The mechanism is unknown, but experiments in which monocytes were genetically engineered suggest that they could help treat Alzheimer's disease.

Multiple studies conclude the varied effect of monocytes. There is no doubt that they are an essential part of the immune system, but research shows that that can have either a beneficial or detrimental effect on some conditions. Though investigation is ongoing, some believe that understanding and targeting monocytes can lead to effective treatments for many diseases.