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[Immunology](https://durgical.com/en/immunology/) · [Immune cells](https://durgical.com/en/immunology/#a-cells)

# Regulatory T cell (Treg, FOXP3)

- Cell

Regulatory T cells are the brakes of the immune system: they calm other lymphocytes and protect the body from autoimmunity.

Updated 2026-11-01

## Key points

- Phenotype: CD4⁺ CD25⁺ (high) FOXP3⁺.
- Natural (thymic) Tregs form in the thymus; induced (peripheral) Tregs form from naive CD4 cells outside it.
- Suppress with IL-10, TGF-β and CTLA-4, and by soaking up IL-2.
- IPEX syndrome: FOXP3 mutation, X-linked; severe enteropathy in early infancy, eczema and type 1 diabetes.
- Tregs are part of peripheral tolerance; tumors can recruit them to escape immune attack.

## What it is

Regulatory T cells are a subset of CD4⁺ T cells defined by high CD25 (the IL-2 receptor α chain) and the transcription factor FOXP3. They suppress the activation and expansion of other T cells, and probably B cells and dendritic cells, through IL-10, TGF-β, CTLA-4 and by consuming IL-2. Without them, self-reactive cells that escape the thymus cause severe autoimmune disease, as in IPEX syndrome.

## Key facts

- FOXP3 is the master transcription factor for Treg development and function.
- CTLA-4 is not unique to Tregs; activated conventional T cells also express it.
- Their balance with Th17 cells is shaped by TGF-β and the cytokine context.

## Function

- Limit the size and length of immune responses.
- Keep self-reactive T cells in check (peripheral tolerance).
- Help maintain tolerance to food and to the normal gut flora.
- Contribute to tolerance of the fetus during pregnancy.

## Markers

- CD4
- CD25 (IL-2Rα)
- FOXP3
- CTLA-4 (CD152)

## Clinical notes

- IPEX: boys with diarrhea from the first months of life, eczema, early type 1 diabetes, thyroiditis and autoimmune cytopenias.
- CTLA-4 deficiency causes autoimmunity and immune dysregulation.
- Checkpoint inhibitors against CTLA-4 remove a brake and can cause colitis, hepatitis and endocrine autoimmunity.
- Increasing Treg function is a research strategy for autoimmunity and transplant tolerance.

## Memory aid

FOXP3 is the fox on guard at the henhouse door of tolerance: if the fox is missing (IPEX), the hens attack their own farm.

## Test yourself

Think of the answer, then tap the question to see it.

## Which three markers define regulatory T cells?

CD4, high CD25 (IL-2 receptor α chain) and the transcription factor FOXP3.

## Which gene is mutated in IPEX syndrome, and how is it inherited?

FOXP3; it is X-linked recessive, so it mainly affects boys.

## Which two cytokines do Tregs use to suppress other cells?

IL-10 and TGF-β.

## Why can a CTLA-4 blocking drug cause colitis?

It removes an inhibitory checkpoint used by Tregs and activated T cells, so self-reactive responses can attack the gut.

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## Related

- [CD4⁺ helper T cell](https://durgical.com/en/immunology/cd4-helper-t-cell/)
- [Immune tolerance and how autoimmunity begins](https://durgical.com/en/immunology/immune-tolerance-and-how-autoimmunity-begins/)
- [IL-10 and TGF-β (the brakes)](https://durgical.com/en/immunology/il-10-and-tgf/)
- [Th1, Th2, Th17, Tfh and Treg](https://durgical.com/en/immunology/th1-th2-th17-tfh-and-treg/)
- [Tumor immunology and checkpoint inhibitors](https://durgical.com/en/immunology/tumor-immunology-and-checkpoint-inhibitors/)

[← All of immunology](https://durgical.com/en/immunology/)

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: [Regulatory T Cells (Tregs)](https://www.immunology.org/public-information/bitesized-immunology/cells/regulatory-t-cells-tregs) (British Society for Immunology, 2024); [21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-3-the-adaptive-immune-response-t-lymphocytes-and-their-functional-types) (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); [Immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome](https://medlineplus.gov/genetics/condition/immune-dysregulation-polyendocrinopathy-enteropathy-x-linked-syndrome/) (MedlinePlus Genetics, U.S. National Library of Medicine, 2024).

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