The dihydroxy-leukotriene, leukotriene B4 (LTB4) is a member of the G protein-coupled receptor (GPCR) family, in a subfamily of GPCRs that includes receptors for chemokines and other chemotactic factors. Recently, a second, lower affinity receptor for LTB4 (BLT2) has also been cloned, with broader ligand specificity for various eicosanoids. LTB4 stimulates neutrophil chemotaxis and secretion but may also affect immunomodulation, contraction of certain smooth muscles via an indirect mechanism and activation of the nuclear transcription factor PPARa (peroxisome proliferation activated receptor alpha). Chemotaxis, the principal effects of LTB4 and related dihydroxy-acids on leukocytes, occurs via activation of BLT1 receptors.