Background Information of Monocyte
Monocytes are the largest physiologically occurring leukocytes found in the peripheral blood film. This subset constitutes 2% to 10% of all leukocytes in the human body. They play multiple roles in immune function. Such roles include: (1) replenishing resident macrophages under normal states, and (2) in response to inflammation signals, monocytes can move quickly (approx. 8–12 hours) to sites of infection in the tissues and divide/differentiate into macrophages and dendritic cells to elicit an immune response.
Monocytes originate from the bone marrow as precursors called monoblasts, bipotent cells that differentiated from hematopoietic stem cells. Monocytes circulate in the bloodstream for about one to three days and then typically move into various tissues in different antomical locations throughout the body. In the tissues, monocytes will then mature into tissue-specific resident macrophages or dendritic cells.
Increased appearance of monocytes in PBF is typically indicative of monocytosis. Monocytosis is a clinical symptom of diseases:
- Severe infection (sepsis)
- Chronic inflammation
- Stress response
- Cushing’s syndrome (hyperadrenocorticism)
- Immune-mediated disease
- Pyogranulomatous disease
- RBC regeneration
- Viral Fever
Reduced appearance of monocytes in PBF is typically indicative of monocytopenia. This is typically due to a general leukopenia condition associated with a deficiency in monocytes
Being the largest WBC in the PBF, it appear to be significantly larger than all other WBCs (lymphocytes, neutrophils, basophils and eosinophils) in size. The cytoplasm is plentiful and stains slightly greyish-blue (synonymous to frosted glass). Their cytoplasm contains irregularly scattered fine purplish granules. They have an irregular kidney/bean-shaped dark purplish stained nucleus with deep indentation. *Note that the staining intensity of monocytes are weaker than lymphocytes. Vacuoles may also be observed in the cytoplasm.