Scientists Discover Immune Sentinel Cells in Skin Hair Follicles

Researchers at the University of California, Riverside have identified specialized immune sentinel cells within skin hair follicles that monitor environmental exposure and microbial presence, offering new insight into how the skin defends against pathogens despite its thick, layered structure.

The discovery, published in Frontiers in Cell and Developmental Biology, reveals that hair follicles contain cells resembling M (microfold) cells — epithelial cells traditionally found in the gut and airway tissues that sample antigens from the external environment. These follicle-associated immune cells appear to serve as localized surveillance hubs, concentrating both environmental material and immune sensing activity in a way that compensates for the skin’s physical barrier properties.

Dr. David Lo, distinguished professor of biomedical sciences at UC Riverside and senior author of the study, explained that unlike the single-layer epithelia of the gut and respiratory tract, which allow direct environmental sampling, the skin consists of multiple stratified layers that form a robust physical barrier. “This raises a long-standing question in immunology: how does the skin efficiently monitor external threats despite its thickness?” Lo said.

The research team proposes that hair follicles act as gateway structures that bridge the external environment with underlying immune systems. Within these niches, M cell-like sentinel cells participate in localized immune responses, particularly to Gram-positive bacteria — a group that includes pathogens responsible for conditions ranging from food poisoning to serious respiratory and skin infections.

Diana Del Castillo, first author of the paper and a graduate student in Lo’s lab, noted that these structures integrate environmental exposure, immune sensing, and potentially neural signaling in a highly localized environment. “Hair follicles may represent a central hub for immune surveillance in the skin,” she said.

The findings build on prior research showing that hair follicles are home to immune cells and beneficial bacteria. A 2021 study by NIH researchers identified the ADAM10-Notch signaling pathway as critical for maintaining the balance between microbial communities and skin immunity in follicles. Disruption of this pathway was shown to cause bacterial imbalance, inflammatory responses, and irreversible hair loss in mice.

Together, these studies suggest that hair follicles are not merely structures for hair growth but active participants in skin immunity, helping to regulate microbial balance and detect potential threats. The UC Riverside team conducted their work in mice, and further research will be needed to determine whether similar mechanisms exist in human skin.

By highlighting the role of hair follicles in immune monitoring, the discovery opens new avenues for understanding skin-related conditions, including infections, inflammatory disorders, and hair loss syndromes, where immune-microbial interactions may play a key part.