Monocytes form 3-9% of WBCs and are elevated in many conditions; most of them are benign:
- Viral infections such as Epstein Barr virus, which is responsible for the development of infectious mononucleosis or “kissing fever.”
- Some parasitic infestations, although it is less commonly affected than eosinophils.
- Sarcoidosis, which is a chronic inflammatory lung and lymph node disease.
- Some fungal infections
All the previous conditions see a mild increase in monocytes count. However, an abnormally high level of monocytes which exceeds tens of thousands per cubic milliliter especially if associated with other increase of other WBCs as neutrophils would point towards a possible malignancy. Leukemias are characterized by the uncontrolled high WBCs count and are divided into two main subtypes:
- Myeloid leukemias: which are characterized by increased levels of neutrophils, basophils, eosinophils, and monocytes.
- Lymphoid leukemias: which are characterized by increased levels of lymphocytes.
Both of these conditions are chronic and may be symptomless except for vague symptoms of fatigue and some abdominal pain owing to the enlarged spleen. On the other hand, acute leukemias only have extremely high levels of abnormal cells called blast cells -which are only found in the bone marrow of healthy adults, in the peripheral blood-. Acute leukemias have the typical symptoms of malignancy, with weight loss, abdominal pain, excessive sweating and fever.