Monocytosis is an increase in the number of monocytes circulating in the blood. 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.
Monocytosis has sometimes been called mononucleosis, but that name is usually reserved specifically for infectious mononucleosis.
Monocytosis often occurs during chronic inflammation. Diseases that produce such a chronic inflammatory 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.
- Obesity (cf. Nagareddy et al. (2014), Cell Metabolism, Vol. 19, pp 821-835)
- Miscellaneous causes: sarcoidosis and lipid storage disease.