What can cause a high monocyte count?

Causes of an elevated monocyte count include leukemia, chronic inflammatory disease, tuberculosis, viral infections and parasitic infections, states MedlinePlus. Monocytes are a type of white blood cell.

Monocytes help the immune system fight infection, according to About.com. These white blood cells have the ability to turn into dendritic cells and macrophages when the immune system detects a foreign substance. Dendritic cells identify foreign substances and communicate with B and T cells to help the body build immunity to a particular substance. Macrophages destroy parasites, bacteria and other organisms.

The percentage of monocytes in the blood is usually determined with a blood differential, reports MedlinePlus. A differential compares the number of each type of white blood cell to the total number of blood cells. The result of this comparison is expressed as a percentage. A normal monocyte level ranges from 2 to 8 percent.

The blood differential also measures the percentage of lymphocytes, neutrophils, basophils and eosinophils in the blood, notes MedlinePlus. Lymphocytes protect the body against infection by manufacturing antibodies, explains the University of Rochester Medical Center. Neutrophils digest fungi and bacteria, providing protection against infectious organisms. Basophils regulate the immune response by producing histamine and other chemicals. Eosinophils destroy cancer cells, kill parasites and participate in allergic reactions.