Monocyte
Monocytus
Also known as: mononuclear phagocyte, blood monocyte
The largest blood leukocyte (≈ 2–10%), with a kidney-shaped nucleus and grey-blue cytoplasm, which circulates for 1–3 days before entering tissues to become macrophages or dendritic cells.
Overview
Monocytes measure ≈ 12–20 µm — the largest cells on a normal blood film. They have a large, indented kidney- or horseshoe-shaped nucleus with fine, lacy chromatin and abundant pale grey-blue, "ground-glass" cytoplasm with fine azurophilic granules and occasional vacuoles. Made in the marrow from granulocyte–monocyte progenitors (monoblast → promonocyte → monocyte) under the influence of M-CSF and GM-CSF, they circulate for roughly 1–3 days; a reserve pool also sits in the splenic red pulp.
Three subsets are recognised in human blood: classical (CD14⁺⁺CD16⁻, ≈ 85–90%), which are recruited to inflamed tissue via the chemokine receptor CCR2 (ligand CCL2/MCP-1); intermediate (CD14⁺⁺CD16⁺); and non-classical (CD14⁺CD16⁺⁺) "patrolling" monocytes that crawl along the endothelium. Once in tissues, monocytes differentiate into macrophages or monocyte-derived dendritic cells, replenishing tissue macrophage populations (especially in the gut and dermis) and dominating chronic inflammation.
- Location
Blood, splenic reservoir, and (after emigration) all tissues.
Function
- Precursors of tissue macrophages and inflammatory dendritic cells.
- Phagocytosis and cytokine production (IL-1, IL-6, TNF-α) in the blood and at sites of inflammation.
- Patrol the vasculature and respond to endothelial injury.
- Physiology
Normal count 0.2–1.0 × 10⁹/L. Monocytes are the main cells of the mononuclear phagocyte system in transit; their recruitment peaks 24–48 hours into an acute inflammatory response, after the neutrophil wave.
Clinical significance
Monocytosis
Chronic infections (tuberculosis, infective endocarditis, brucellosis, syphilis), recovery from neutropenia, chronic inflammatory disease (IBD, sarcoidosis), and haematological malignancy (chronic myelomonocytic leukaemia, monocytic AML).
Atherosclerosis
Monocytes enter the arterial intima at sites of endothelial dysfunction, become macrophages, ingest oxidised LDL and turn into lipid-laden foam cells — the hallmark of the fatty streak.
Compiled from standard anatomy and physiology references — see the Lymphatic & Immune System sources. For education only; not medical advice.