Monocytes are white blood cells in the immune system. As part of the body's immune function, monocytes help get rid of harmful substances, dead cells and cancer cells. This means that infections and cancers tend to raise blood monocyte levels. Certain blood disorders, genetic disorders and autoimmune diseases are also associated with increased monocyte levels. A person's monocytes count can also fall, and this can be caused by the use of steroid drugs, chemotherapy treatments or toxins produced by some bacteria.
Levels of monocytes may be measured as part of a test known as a white blood cell differential count. This calculates the percentages of the different kinds of white cell in a blood sample. Monocytes normally represent around five to ten percent of the total white cell count.
Along with other blood cells, monocytes are formed in bone marrow, so disorders which damage bone marrow, such as cancer, can cause low levels of monocytes. Normally, monocytes travel in the bloodstream to different parts of the body where they develop into phagocytic cells called dendritic cells and macrophages. Phagocytic cells are scavengers, able to consume and destroy harmful particles such as bacteria and waste matter such as dead cells. This is why levels of monocytes are elevated in response to infection and inflammation -- so they can remove microbes and dead cells from tissues. Tuberculosis and syphilis are two examples of bacterial diseases which cause raised monocyte levels, and viruses such as measles and mumps can have a similar effect.
Malignant conditions such as leukemia or lung cancer can lead to increased monocyte levels, along with raised levels of other types of white blood cells. In leukemia, the bone marrow becomes cancerous and begins to produce large numbers of monocytes. Autoimmune conditions, where the immune system mistakenly attacks the body's own tissues, are also associated with increased monocyte levels. Such conditions include rheumatoid arthritis, which affects the joints, and inflammatory bowel disease.
Certain bacteria, such as E.coli, produce particles known as endotoxins. Endotoxins can cause endotoxic shock, a potentially fatal condition in which low blood pressure, organ failure and hemorrhages occur. The condition is also associated with decreased levels of monocytes and other white cells.
For patients who have an abnormally high monocyte count, management usually involves treating the underlying cause. Low numbers of monocytes and other white cells can sometimes leave patients vulnerable to infections. It may be necessary to stop any drugs causing the problem. Medications are also available that can increase white cell levels.