MR1 (major histocompatibility complex, class I-related) is a non-classical MHC class I-like molecule that presents small molecule antigens to specialized T cell populations, particularly mucosal-associated invariant T (MAIT) cells 1. Unlike classical MHC molecules that present peptides, MR1 binds and presents vitamin metabolites, specifically riboflavin (vitamin B2) and folic acid (vitamin B9) derivatives derived from microbial biosynthetic pathways 2. The structure of MR1's antigen-binding cleft is distinct from MHC and CD1 families, being ideally suited to sequester vitamin metabolites such as 6-formyl pterin 2. MR1 is highly conserved across mammals and exhibits limited surface expression, instead residing primarily in intracellular vesicles and the endoplasmic reticulum 3. Since mammals cannot synthesize riboflavin, MR1 functions as a sensor of the microbial metabolome, enabling early detection of intracellular infections 3. The MR1-MAIT cell axis represents a novel paradigm in antigen presentation, allowing immunosurveillance through recognition of metabolites unique to bacteria and yeast 2. This system provides a mechanism for the immune system to detect microbial presence through metabolic signatures rather than traditional peptide antigens 4.