Jeffery and colleagues characterized liver-infiltrating mucosal-associated invariant T (MAIT) cells and their role in biliary immune surveillance. They found that these cells predominantly localize to bile ducts in the portal tracts, as well as being present in the hepatic sinusoids. Interestingly, MAIT cells upregulated IFNγ and CD40 ligands and degranulated in an MR1-dependent, cytokine-independent manner in response to macrophages, biliary epithelial cells and liver B cells that had been exposed to Eschericia coli. These findings are evidence of an immune surveillance effector response of MAIT cells to biliary epithelial cells in human liver.