What Are M Cells and What Is Their Function?

M cells, short for microfold cells, are specialized epithelial cells whose primary job is to sample material from the body’s mucosal surfaces and hand it off to the immune system waiting below. They sit in the lining of the gut and other mucosal tissues, acting as sentries that grab bacteria, viruses, and other particles from the outside world and shuttle them across the epithelial barrier so immune cells can inspect them and mount a response. Without M cells, the immune system would be largely blind to what is happening in the gut lumen and other mucosal environments. That surveillance role makes them essential for protective immunity, but it also creates a vulnerability that certain pathogens have learned to exploit.

Where M Cells Live

M cells were first identified in the gut-associated lymphoid tissue, specifically in structures called Peyer’s patches, which are clusters of immune tissue scattered along the wall of the small intestine. The epithelium that covers a Peyer’s patch is known as the follicle-associated epithelium, and M cells are a distinct cell type within it. They were originally assumed to be confined to the gut, but researchers have since found them in mucosal tissues throughout the body, including the colon, nasal passages, lungs, eyes, and even the thymus.¹ Wherever they appear, the pattern is the same: M cells sit in the epithelium directly above organized lymphoid tissue, positioning themselves at the exact interface between the outside environment and the immune cells that need to be alerted.

The name “microfold” comes from their unusual surface. Most intestinal epithelial cells have a dense brush border of tall, finger-like projections called microvilli. M cells have shorter, irregular folds instead, which gives them a flatter apical surface. That reduced brush border is not a defect. It makes the cell’s surface more accessible to particles and microorganisms in the gut lumen, essentially creating a landing pad for anything floating by.

How M Cells Sample the Gut

The defining function of an M cell is transcytosis: grabbing material from the lumen side, pulling it into the cell, packaging it in a vesicle, and releasing it on the other side where immune cells are clustered. This happens quickly. The M cell’s thin cytoplasm and relatively sparse organelles create a short transit path, so antigens can cross the barrier within minutes. On the basal side, M cells have a deep pocket where dendritic cells and lymphocytes nestle in close, ready to receive whatever the M cell delivers.² This architecture is remarkably efficient: the immune cell does not have to go searching for the antigen, because the M cell deposits it directly into the cell’s embrace.

M cells are not passive funnels. They carry surface receptors that recognize specific molecular signatures on microorganisms. Among the receptors identified so far are toll-like receptor 4, the platelet-activating factor receptor, and the integrin α5β1, all of which help M cells bind and internalize bacterial components.³ Another receptor, glycoprotein 2 (GP2), is found exclusively on M cells among intestinal epithelial cells and acts as a grabbing mechanism for bacteria that display a surface protein called FimH. FimH is a common feature of gram-negative bacteria such as E. coli and Salmonella enterica, so GP2 gives M cells a way to selectively pull in some of the most relevant microbial targets and trigger bacteria-specific immune responses.⁴

This selectivity matters. The gut lumen contains trillions of microorganisms plus a constant flow of food particles and debris. M cells are not sampling everything indiscriminately; they are biased toward particles that carry the molecular patterns associated with potential threats. That bias helps focus the immune response where it is most needed.

How M Cells Develop

M cells differentiate from the same precursor cells that give rise to ordinary intestinal epithelial cells, but they require a specific molecular signal to commit to the M cell fate. That signal is RANKL, a protein produced by stromal cells in the tissue beneath the follicle-associated epithelium. When RANKL binds its receptor on epithelial precursors, it activates a transcription factor called Spi-B, which drives the cell toward M cell identity. Mice that lack the gene for Spi-B completely fail to produce M cells in their Peyer’s patches, confirming that Spi-B is not just helpful but required.⁵ When researchers administered RANKL to mice experimentally, it triggered M cell differentiation not just in Peyer’s patches but across all the small intestinal villi, producing villous M cells capable of actively taking up bacteria and particles.⁶

The timing of M cell development turns out to be linked to the gut microbiota itself. Research in mice has shown that the differentiation of M cells within the follicle-associated epithelium is necessary for the maturation of a key class of immune cells in Peyer’s patches, and that this process is set in motion around weaning, when the microbial community in the gut undergoes major shifts. Flagellated bacteria that bloom during the early postnatal period appear to play a stimulatory role, and M cell development was both required and sufficient to initiate the downstream immune cell maturation.⁷ In other words, M cells do not just passively respond to antigens; their very appearance during early life helps kick-start the intestinal immune system.

Villous M Cells Beyond Peyer’s Patches

For a long time, M cells were thought to exist only in the follicle-associated epithelium overlying organized lymphoid tissues like Peyer’s patches. That picture has changed. Researchers have identified clusters of M-cell-like cells on ordinary intestinal villi, away from any Peyer’s patch, and dubbed them villous M cells. These cells share key markers with their Peyer’s patch counterparts and can take up bacteria, including Salmonella, Yersinia, and invasin-expressing E. coli, and deliver bacterial antigens to immune cells in a way that triggers antigen-specific immune responses.⁸

Villous M cells have been found even in mice that completely lack Peyer’s patches and other organized gut lymphoid tissues, which means they develop through a pathway that does not depend on those structures. This gives the intestine a second, independent gateway for antigen sampling. The practical significance is that even when organized lymphoid tissue is absent or impaired, the gut retains some ability to survey its microbial contents and generate immune responses through the mucosal compartment.

How Pathogens Exploit M Cells

The same transport machinery that makes M cells valuable to the immune system can be turned against the host. Because M cells are designed to pull in microorganisms and move them across the epithelial barrier, pathogens that target M cells get a free ride into the tissue beneath.⁹ Different pathogens have evolved different strategies for doing this, and the consequences for the host range from contained infection to severe tissue destruction.

Salmonella Typhimurium is one of the best-studied examples. It preferentially targets M cells to cross the gut epithelium, and infection actually causes the number of M cells to increase. The mechanism behind this is striking: Salmonella secretes a protein called SopB, which activates a signaling pathway in neighboring epithelial cells and effectively transforms ordinary enterocytes into M cells. This cellular conversion creates more entry points for the bacterium, accelerating invasion.¹⁰ It is a remarkable bit of microbial manipulation: rather than simply exploiting the M cells that already exist, Salmonella manufactures new ones.

Other enteric pathogens follow their own playbooks. Studies comparing Salmonella, Listeria monocytogenes, and Shigella flexneri have found that all three enter through M cells, but they differ in how much damage they cause along the way. Salmonella has a highly efficient entry mechanism that destroys the M cells it passes through. Listeria and Shigella enter in a somewhat less disruptive fashion at first, but at later stages of infection, the follicle-associated epithelium of animals infected with these bacteria shows extensive destruction, with visible holes appearing in the intestinal lining.¹¹

The vulnerability is not limited to bacteria. Prion diseases, which are caused by misfolded proteins rather than living organisms, also depend on M cells to gain entry from the gut. When researchers depleted M cells in mice before exposing them to prions through the oral route, the usual chain of events was completely broken: prions failed to accumulate in Peyer’s patches, failed to spread to the nervous system, and the animals never developed disease.¹² Conversely, mice engineered to have an increased abundance of M cells in their gut epithelium showed dramatically enhanced susceptibility to oral prion infection.¹³ The number of M cells, in other words, acts almost like a dial controlling how easily prions can cross the gut barrier.

M Cells During Inflammation and Aging

M cell numbers are not fixed. They shift in response to the body’s state, and these shifts have consequences for both immunity and disease susceptibility. During intestinal inflammation, the colonic epithelium can develop new populations of M cells that were not present in healthy tissue. In mouse models of colitis, researchers observed significantly increased numbers of M cells in colonic epithelium at the peak of inflammation, often alongside loosely organized immune cell clusters in the tissue beneath.¹⁴ Whether these inflammation-induced M cells are protective (boosting immune surveillance when the barrier is under stress) or harmful (creating new entry points for bacteria in already damaged tissue) is not fully settled, and likely depends on context.

Aging tells a different but equally interesting story. In aged mice, M cells in Peyer’s patches show dramatically reduced functional maturation compared to those in young animals. The surprise is that the signals telling M cells to develop appear to be fine: levels of RANKL and its receptor are no different between young and old mice.¹⁵ Something downstream of the initial signal is going wrong, so the cells are getting the instruction to differentiate but are failing to fully mature into functional antigen-sampling cells. This could help explain why mucosal immune responses tend to weaken with age, and why older adults often respond poorly to oral vaccines, since those vaccines depend on M cells to deliver antigens to the gut immune system.

Why M Cells Are Hard to Study

One persistent frustration in M cell research is that these cells are rare and difficult to identify reliably. They make up a small fraction of the follicle-associated epithelium, and there is no single universal marker that unambiguously identifies an M cell in every species and tissue. Different studies have used different combinations of markers (GP2, certain lectin-binding patterns, Spi-B expression, uptake of fluorescent particles), and a thorough evaluation of the literature has found that many studies lack consistent and universally recognized methods for identifying M cells in their experimental models.¹⁶ This is not just a technical nuisance. It means that comparing results across labs, across species, and between in vitro models and living tissue is harder than it should be.

Most of what we know about M cell biology comes from mouse studies, and while the broad principles appear to hold in humans, the specific molecular details do not always translate cleanly. Human M cells are difficult to access, and biopsy samples from Peyer’s patches are small and rare. In vitro models that try to mimic M cells using co-cultures of intestinal cell lines are useful but imperfect: they reproduce some features of M cell behavior but not all. Researchers have called for standardized criteria to define what counts as an M cell in a given model, and progress toward that goal has been slow.

Vaccine and Drug Delivery Applications

The fact that M cells are nature’s built-in transport system across mucosal barriers makes them attractive targets for oral vaccine design. The logic is straightforward: if you can get a vaccine antigen to M cells and ensure it is taken up efficiently, you can trigger mucosal immune responses without needing an injection. This is especially appealing for diseases caused by pathogens that enter through mucosal surfaces, because mucosal immunity (particularly the production of secretory antibodies at the site of entry) is something that injected vaccines often fail to generate well.¹⁷

Several approaches are being explored. One strategy involves coating nanoparticles with ligands that M cells recognize. For instance, nanoparticles decorated with molecules that bind integrins on the M cell surface can significantly increase transport across in vitro M cell models compared to non-targeted particles, and some of these formulations have produced stronger immune responses in animals after oral administration than either free antigen or non-targeted nanoparticles.¹⁸ In experiments using chitosan-based nanoparticles, transport through an M cell co-culture model was about five-fold higher than through a standard intestinal epithelial monolayer, and the particles decorated with targeting ligands showed an additional three- to five-fold boost in transport efficiency.¹⁹

The challenge is moving from promising lab results to reliable oral vaccines in humans. M cells are rare, the gut environment is harsh (stomach acid, digestive enzymes, mucus), and getting enough antigen to M cells to generate a strong immune response remains difficult. But the potential payoff is large: effective oral vaccines would be easier to distribute, would not require trained healthcare workers for injection, and could provide mucosal protection at the very sites where infections start.

An Ancient Cell Type

M cells are not a recent evolutionary invention of mammals. Structures resembling mucosal lymphoid tissue, complete with cells displaying M-cell-like characteristics, have been identified in the African lungfish, a species whose lineage diverged from the mammalian line hundreds of millions of years ago. Cells with a goblet-cell-like shape that stained positive for an M-cell-associated lectin marker were found in the lungfish mucosa.²⁰ This suggests that the strategy of embedding specialized antigen-sampling cells in mucosal epithelia is ancient, predating the evolution of the mammalian adaptive immune system as we know it. The basic architectural principle of placing a sampling cell at the boundary between an organism and its microbial environment appears to have been so successful that it has been retained and refined across vertebrate evolution.

The presence of M-cell-like features in such a distant relative also implies that the partnership between mucosal epithelial sampling and organized lymphoid tissue may have coevolved from the start. Rather than being an add-on that mammals developed independently, the M cell niche appears to be a fundamental component of how vertebrates have managed their relationship with the microbial world from early in their evolutionary history.