Monocytosis

Monocytosis is an increase in the number of monocytes circulating in the blood.^[1] Monocytes are white blood cells that give rise to macrophages and dendritic cells in the immune system.

In humans, 950/μL is regarded as at the upper limit of normal; monocyte counts above this level are regarded as monocytosis.

Causes

Monocytosis often occurs during chronic inflammation. Diseases that produce this state:

• Infections: tuberculosis, brucellosis, listeriosis, subacute bacterial endocarditis, syphilis, and other viral infections and many protozoal and rickettsial infections (e.g. kala azar, malaria, Rocky Mountain spotted fever).
• Blood and immune causes: chronic neutropenia and myeloproliferative disorders.
• Autoimmune diseases and vasculitis: systemic lupus erythematosus, rheumatoid arthritis and inflammatory bowel disease.
• Malignancies: Hodgkin's disease and certain leukaemias, such as chronic myelomonocytic leukaemia (CMML) and monocytic leukemia.
• Recovery phase of neutropenia or an acute infection.
• Miscellaneous causes: sarcoidosis and lipid storage disease.

References