Immunology · Cytokines and molecules
Interleukin-6 !
- Molecule
IL-6 tells the liver to make CRP and other acute-phase proteins. Blocking its receptor is a treatment for rheumatoid arthritis and cytokine storms.
Updated 2026-11-01
What it is
Interleukin-6 is made mainly by macrophages and other myeloid cells, and also by T cells, fibroblasts and fat tissue. It acts through the IL-6 receptor plus the signalling chain gp130 (classic signalling on cells with membrane IL-6R, trans-signalling through soluble IL-6R on almost any cell). It is the main trigger of the hepatic acute-phase response and links innate inflammation with B-cell and Th17 responses.
Function
- Acute-phase response and fever, together with IL-1 and TNF-α.
- Recruitment of neutrophils and monocytes.
- Differentiation of B cells into antibody-secreting plasma cells.
- Decides between Th17 (IL-6 present) and regulatory T cells (TGF-β alone).
Clinical notes
- Rheumatoid arthritis, giant cell arteritis, juvenile idiopathic arthritis and systemic sclerosis lung disease: tocilizumab.
- Cytokine release syndrome (after CAR-T cells) and severe COVID-19 needing oxygen: tocilizumab.
- Before treatment: screening for latent tuberculosis and hepatitis B, because infections may become severe.
- Known effects of IL-6 receptor blockade: neutropenia, raised lipids, liver enzyme changes and, rarely, bowel perforation (especially with diverticulitis).
In the lab
- CRP rises within hours of inflammation because of IL-6; it falls quickly once IL-6 signalling stops.
- Low CRP under tocilizumab does not rule out infection.
Test yourself
Think of the answer, then tap the question to see it.
Which cytokine is the main driver of CRP synthesis?
IL-6, acting on hepatocytes through IL-6R/gp130 and STAT3.
How does IL-6 cause the anaemia of inflammation?
It induces hepcidin, which blocks iron release from macrophages and gut cells, so less iron reaches the marrow.
Why can infection be hidden in a patient on tocilizumab?
Blocking the IL-6 receptor suppresses CRP and fever, so the usual signs of infection may be absent.
Related
- Cytokines: the table to know
- Interleukin-1
- TNF-α
- Interleukin-17
- IL-10 and TGF-β (the brakes)
- Acute-phase response and fever (CRP, ESR, procalcitonin)
- Th1, Th2, Th17, Tfh and Treg
- Monoclonal antibodies and biologics: naming and targets
Related conditions
In Pharmacology
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: Update on tocilizumab in rheumatoid arthritis: a narrative review (Frontiers in Immunology, 2025); Tocilizumab Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2025); Pro-Inflammatory and Anti-Inflammatory Interleukins in Infectious Diseases: A Comprehensive Review (Tropical Medicine and Infectious Disease (PubMed Central), 2024); Regulation of Treg cells by cytokine signaling and co-stimulatory molecules (Frontiers in Immunology, 2024).
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Representative model, for educational purposes only.