1. Sense microbes
Recognize metabolites from many bacteria and fungi with riboflavin synthesis pathways.
MAIT Cells
Mucosal-Associated Invariant T Cells
Microbial Sensors. Barrier Defenders.
MAIT cells are innate-like T lymphocytes that recognize microbial metabolites presented by MR1. They respond rapidly to infection and help coordinate tissue immunity.
Back to all immune cells →What Are MAIT Cells?
MAIT cells express a semi-invariant αβ T-cell receptor. Unlike conventional T cells that usually recognize peptides, they detect small microbial metabolites displayed by MR1, an MHC-related molecule.
Many activating antigens arise from microbial riboflavin (vitamin B2) synthesis. MR1 differs from classical MHC and from CD1d, the lipid-presenting molecule recognized by NKT cells.
MAIT cells at a glance
Type of cell
Innate-like T lymphocyte
Recognition
MR1-presented microbial metabolites
Where they are
Blood, liver, gut and lungs
Activation
TCR signals or inflammatory cytokines
Key Functions of MAIT Cells
MAIT-cell responses vary with antigen exposure, cytokines and tissue context.
2. Release immune signals
Produce IFN-γ, TNF and, in some settings, IL-17.
3. Kill infected targets
Activated MAIT cells can use cytotoxic molecules to eliminate bacterially exposed cells.
4. Tissue repair potential
Human laboratory studies identify repair-associated programs after TCR activation.
How MAIT Cells Work
From microbial metabolites to a coordinated cellular response.
1. Microbial metabolism
Bacteria and fungi produce riboflavin-pathway metabolites.
2. MR1 presentation
An antigen-presenting cell displays a metabolite bound to MR1.
3. TCR recognition
The MAIT TCR contacts the MR1–metabolite complex.
4. Response
Activated cells release cytokines and can deploy cytotoxic molecules.
A second activation route
IL-12 + IL-18 → cytokine-driven activation
IL-12 and IL-18 can activate MAIT cells without TCR engagement. This helps explain responses during viral infections: viruses do not synthesize riboflavin.
MAIT Cells in Health and Disease
- Barrier defense: help respond to microbes at tissue interfaces.
- Inflammation: activation can protect or contribute to injury, depending on context.
- Tissue repair: promising mechanistic evidence; clinical benefit remains under investigation.
MAIT Cells and Aging
- Human studies report lower circulating MAIT-cell numbers in older adults.
- Phenotype and function change with age; blood measurements may not reflect tissue populations.
- MAIT-cell counts alone are not a validated test of immune age.
Important distinction
Recognition of microbial vitamin B2 metabolites does not mean vitamin B2 supplements boost MAIT cells.
Selected sources and references
- Kjer-Nielsen et al., 2012 — MR1 presents microbial vitamin B metabolites
- Ussher et al., 2014 — IL-12/IL-18 activation without TCR signaling
- Ussher et al., 2014 — MAIT cells in antibacterial immunity
- Leng et al., 2019 — TCR and inflammatory signals tune tissue-repair functions
- Kurioka et al., 2015 — MAIT-cell cytotoxic licensing
- Novak et al., 2014 — MAIT-cell numbers and phenotype with age
Artwork illustrates the main mechanisms schematically; molecular sizes, colors and distances are not literal.