Monocyte and macrophage !
- Cell
Monocytes circulate in blood and become macrophages in tissues: big eaters that clean up, kill germs, signal alarm and show antigens to T cells.
Updated 2026-11-01
What it is
Monocytes are the largest white cells in blood, with a kidney- or horseshoe-shaped nucleus. When they enter tissues they mature into macrophages, long-lived phagocytes that remove microbes, dead cells and debris and release cytokines that drive inflammation. Activated by helper T cells, they wall off hard-to-kill microbes such as Mycobacterium tuberculosis inside granulomas.
Key facts
- Origin: myeloid progenitor in the bone marrow; some tissue macrophages settle before birth and renew locally.
- Lifespan: monocytes circulate for a short time; tissue macrophages live much longer than neutrophils.
- They are professional antigen-presenting cells (MHC class II) but weaker than dendritic cells at starting new T-cell responses.
Function
- Phagocytosis of microbes, dead cells and old red cells.
- Killing with reactive oxygen species and nitric oxide.
- Recognition of microbes through Toll-like receptors and cytokine release.
- Tissue repair and clean-up after inflammation.
Markers
- CD14
- CD16 (some monocytes)
- CD68
- MHC class II
- CD80/CD86 when activated
Clinical notes
- Monocytosis: tuberculosis, chronic inflammatory disease, some viral and parasitic infections and some leukemias.
- Tuberculosis, a public health priority in Colombia, is the classic granulomatous infection of macrophages.
- Defects in the IFN-γ and IL-12 pathway cause severe mycobacterial infections.
- Leishmania and Histoplasma also live inside macrophages.
In the lab
- Absolute monocyte count in the differential.
- Tissue biopsy: granulomas with epithelioid macrophages and giant cells; stains for acid-fast bacilli.
Test yourself
Think of the answer, then tap the question to see it.
What is a monocyte called once it settles in the liver, and in the brain?
A Kupffer cell in the liver and a microglial cell in the brain.
Which cytokine from Th1 cells activates macrophages?
Interferon-γ.
Why can anti-TNF drugs reactivate latent tuberculosis?
TNF-α is needed to keep granulomas intact; blocking it lets contained mycobacteria escape.
Which three macrophage cytokines cause fever and the acute-phase response?
IL-1, IL-6 and TNF-α.
Related
- Neutrophil
- Dendritic cell (professional antigen-presenting cell)
- Granulomatous inflammation
- Interferon-γ
- TNF-α
- Antigen processing and presentation
- CD markers: the table to know
Related conditions
Microorganisms
Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.
Sources: 18.4 Leukocytes and Platelets (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); 21.2 Barrier Defenses and the Innate Immune Response (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); Macrophages (British Society for Immunology, 2024); Blood differential test (MedlinePlus, U.S. National Library of Medicine, 2024).
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Representative model, for educational purposes only.