# T Cell Subsets and Functions

> T cell subsets for NEET-PG Microbiology covering Th1, Th2, Th17, regulatory and cytotoxic T cells, with signature cytokines and transcription factors.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/t-cell-subsets-and-functions
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Abbas, Lichtman and Pillai Cellular and Molecular Immunology; Janeway's Immunobiology; Ananthanarayan and Paniker's Textbook of Microbiology
- How to cite: "T Cell Subsets and Functions", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/t-cell-subsets-and-functions

## Direct answer

T cell subsets are functionally distinct lymphocyte populations, identified by surface markers, transcription factors and the cytokines they secrete. CD4 helper cells differentiate into Th1, Th2, Th17 and regulatory T cells, each tuned to a different class of threat, while CD8 cytotoxic cells kill infected or malignant cells. Naive T cells need three signals from an antigen presenting cell before they become effectors.

## What you must remember

- **Th1 cells:** IL-12-driven CD4 cells that depend on T-bet and secrete IFN-γ, which activates macrophages against intracellular pathogens such as Mycobacterium tuberculosis.
- **Th2 cells:** GATA-3-dependent cells secreting IL-4, IL-5 and IL-13, which drive IgE class switching, eosinophilia and defence against helminths.
- **Th17 cells:** RORγt-dependent cells producing IL-17 and IL-22, important against extracellular bacteria and fungi such as Candida.
- **Regulatory T cells:** CD4+ CD25+ FoxP3+ cells that suppress effector responses through IL-10, TGF-β and CTLA-4, with FoxP3 mutations causing IPEX syndrome.
- **Cytotoxic T cells:** CD8+ cells that recognise antigen on MHC class I and kill target cells through perforin and granzyme release.
- **MHC restriction:** CD4 cells recognise peptide on MHC class II, whereas CD8 cells recognise peptide on MHC class I.
- **Co-stimulation:** CD28 on the T cell binding B7 on the antigen presenting cell supplies the second signal that prevents anergy.
- **Thymic selection:** positive selection occurs in the cortex and negative selection in the medulla, where AIRE helps display tissue antigens to remove autoreactive clones.

## From naive cell to effector: the differentiation pathway

A naive T cell does not pick a subset at random, because the cytokines around it at the moment of activation decide the fate. The steps below follow the sequence most textbooks use to explain it.

1. **Antigen recognition:** a naive CD4 cell binds peptide on MHC class II of a dendritic cell through its T cell receptor, which provides the first signal.
2. **Co-stimulation:** CD28 engages B7, supplying the second signal. Without it the T cell becomes anergic instead of expanding.
3. **Cytokine instruction:** IL-12 from dendritic cells and macrophages pushes the cell towards Th1, IL-4 pushes it towards Th2, and IL-6 with TGF-β, sustained by IL-23, pushes it towards Th17.
4. **Lineage commitment:** the instructing cytokines switch on master regulators, so T-bet marks Th1, GATA-3 marks Th2, RORγt marks Th17 and FoxP3 marks regulatory cells.
5. **Effector function:** each committed cell secretes its signature cytokines and recruits the right effector cells, such as macrophages, eosinophils or neutrophils.
6. **Memory and contraction:** most effector cells die once the antigen is cleared, and a smaller pool persists as memory cells for a faster secondary response.

## Where the exam tries to catch you

Subset questions usually turn on which marker, cytokine or transcription factor goes with which lineage, so precise pairing wins marks.

- **Trap:** assuming CD4 and CD8 cells both recognise antigen on MHC class II — **Correct:** CD4 cells use MHC class II and CD8 cells use MHC class I, which is why the marker tracks the helper or cytotoxic role.
- **Trap:** treating FoxP3 as a marker of Th17 cells — **Correct:** FoxP3 identifies regulatory T cells, whereas RORγt identifies Th17 cells, so the transcription factor is the reliable label.
- **Trap:** linking IgE production to Th1 cells — **Correct:** IL-4 from Th2 cells drives IgE switching, so Th2 is the subset behind allergy and helminth immunity.

## Frequently asked questions

### Which transcription factor defines Th17 cells?
RORγt defines Th17 cells, and it drives production of IL-17 and IL-22. Th17 cells are important against extracellular bacteria and fungi, and they are also linked to autoimmune inflammation when left unchecked.

### Which cytokine pushes naive CD4 T cells towards the Th1 subset?
IL-12, released mainly by dendritic cells and macrophages, pushes naive CD4 T cells towards the Th1 subset. Th1 cells then secrete IFN-γ, which activates macrophages and supports killing of intracellular organisms such as Mycobacterium tuberculosis.

### How do regulatory T cells suppress immune responses?
Regulatory T cells suppress immune responses through IL-10 and TGF-β, through consumption of IL-2 and through CTLA-4 signalling. Their development depends on FoxP3, so a FoxP3 mutation causes IPEX syndrome, a disorder of autoimmunity, enteropathy and endocrine failure.

### Why do helper T cells carry the CD4 marker rather than CD8?
Helper T cells carry CD4 because CD4 binds MHC class II, which presents peptides from extracellular antigens to the T cell. Cytotoxic T cells carry CD8 instead, since CD8 binds MHC class I, the molecule that displays peptides from infected or malignant cells.