G protein-coupled receptors (GPCRs) comprise one of the largest collections of transmembrane proteins in the mammalian genome. Recently, a subfamily of G protein coupled receptors have been identified that utilize short chain free fatty acids as ligands, FFA1 (GPR40), FFA2 (GPR43) and FFA3 (GPR41). Research has intimated that these receptors are involved in essential biological crosstalk between colonic bacteria, host and immune defenses. Although highly conserved in structure, members of this sub family demonstrate differences in fatty acid specificity, intracellular signaling mechanisms and tissue localization. Specifically, FFA2 (GPR43)4 expression has been shown to be expressed in higher magnitudes within immune cells, has been hypothesized to be involved in the modulation of pro- and anti-inflammatory mediators, such as prostaglandin E2 and is involved in neutrophil chemotaxis in mice. GPR43 is involved in the process of differentiation of progenitor immune cells into monocytes and macrophages, a key component of host defense. In addition, highly selective expression of GPR43 in polymorphonuclear cells point to strong role in chemotaxis and recruitment of immune cells to foci of bacterial infection. Short chain fatty acids also activate GPR43 receptors on intestinal epithelial cells with rapid production of secondary chemokine and cytokines after being challenged with inflammation and infection, and likely aid in the coordinated immune response. GP43 is likely an essential component of the phagocytic chemotaxis that maintains normal intestinal immune defense. In a model of acute colitis, GPR43 knockout mice demonstrated increased mortality. The authors concluded that GPR43 deficiency likely suppressed the role of immune cells within the gut immune milieu leading to death from sepsis related complications. Conversely, in a chronic colitis model, GPR43 deficient knockout mice demonstrated reduced colonic inflammation, without overwhelming infection, the authors concluded that GPR43 has an unfavorable role in the manifestation of chronic inflammatory conditions. These findings were confirmed with at least one later study. In humans, fetal membrane expression of GPR43 was higher in parturients in preterm labor with signs of infection.