Flagellin is the major structural protein monomer of bacterial flagella. Flagellin through binding to its receptor and activation of antigen presenting cells stimulates the innate and adaptive immune responses. Flagellin is used as an effective systemic or mucosal adjuvant to stimulate the immune system. Flagellin is an agonist of Toll-like receptor 5 (TLR5), a pattern recognition receptor (PRR) of the innate immune system expressed on the basolateral surface of intestinal epithelial cells and on the surface of a subset of intestinal dendritic cells. Flagellin is delivered into the cytosol of macrophages by the T3SS-1 of serotype Typhimurium, where it activates the cytosolic interleukin-1 (IL-1) converting enzyme-protease activating factor (IPAF), a nucleotide-binding and oligomerization domain-like receptor (NLR) of the innate immunesystem. Recognition of flagellin by IPAF leads to activation of the inflammasome, followed by proteolytic activation of IL-1 and IL-18.