Step by Step
Th1
Th1 — fights intracellular pathogens
Induced by IL-12, Th1 cells secrete IFN-γ, which activates macrophages to kill intracellular bacteria (like TB and Listeria) and viruses.
Th2
Th2 — fights parasites (and drives allergy)
Induced by IL-4, Th2 cells secrete IL-4, IL-5, and IL-13, activating eosinophils and mast cells and driving IgE production — this subset fights parasites and helminths, but is also responsible for allergic responses.
Th17
Th17 — fights extracellular bacteria and fungi
Induced by IL-6 plus TGF-β, Th17 cells secrete IL-17, recruiting neutrophils to fight extracellular bacteria and fungi. This subset is also implicated in autoimmune diseases like psoriasis, IBD, and rheumatoid arthritis.
Treg
Treg — suppresses immune responses
Regulatory T cells, identified by the transcription factor FoxP3, secrete IL-10 and TGF-β, suppressing immune responses to prevent autoimmunity and limit collateral tissue damage from an ongoing immune response.
Applied Walkthrough
1
A patient with a tuberculosis infection mounts a Th1-dominant response: IL-12 induces Th1 differentiation, and the resulting IFN-γ secretion activates macrophages to fight this intracellular bacterial pathogen.
2
A different patient with a parasitic worm infection instead relies on a Th2-dominant response: IL-4 induces Th2 differentiation, and the resulting cytokines activate eosinophils and drive IgE production to combat the parasite.
3
A patient with psoriasis has an overactive Th17 response — the IL-17 these cells secrete, meant to recruit neutrophils against extracellular bacteria and fungi, instead contributes to the chronic inflammatory skin condition when dysregulated.
4
Meanwhile, Treg cells work in the background across all these scenarios, secreting IL-10 and TGF-β to keep each of these immune responses appropriately contained, preventing them from spiraling into autoimmune damage against the body's own tissues.
Exam Application
Exams test whether you can match each T helper subset (Th1, Th2, Th17, Treg) to its inducing cytokine, its secreted cytokines, and the type of pathogen (or regulatory role) it specializes in.
⚠ Common Trap
The most common trap is confusing Th2's antiparasitic role with Th17's antibacterial role — Th2 fights parasites and drives allergic responses (via IgE, eosinophils, mast cells), while Th17 fights extracellular bacteria and fungi (via neutrophil recruitment) — these are distinct pathogen categories and distinct cytokine profiles.
✓ Quick Self-Check
1. What cytokine induces Th1 differentiation, and what does Th1 fight?
IL-12; Th1 fights intracellular bacteria and viruses.
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2. What cytokine induces Th2 differentiation, and what does Th2 fight?
IL-4; Th2 fights parasites and helminths, and also drives allergic responses.
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3. What does Th17 fight, and what cytokine does it secrete to do so?
Extracellular bacteria and fungi; it secretes IL-17, recruiting neutrophils.
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4. What is the function of Treg cells, and what transcription factor identifies them?
Suppressing immune responses to prevent autoimmunity; identified by FoxP3.
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5. Name two autoimmune diseases associated with Th17 dysregulation.
Any two of: psoriasis, IBD, rheumatoid arthritis.
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