HLA-DR expression on M cells overlying Peyer's patches is a common feature of human small intestine.

The expression of major histocompatibility complex (MHC) class II (HLA-DR) antigens on M cells within follicle-associated epithelial cells (FAE) covering Peyer's patches was analyzed immunohistochemically. HLA-DR antigens were strongly expressed on M cells and cells with dendritic morphology, whereas other FAE covering Peyer's patches showed weak, but definite staining. Dendritic cells were also positive for S-100 protein and IL-1. These findings suggest that M cells as well as dendritic cells within FAE form an antigen-presenting cell system and contribute to the first information releaser for the mucosal immune system.