In contrast, the 15-hydroxy residues of the two epi-LXs are 15"R" chirality products because they are synthesized by aspirin-treated cyclooxygenase 2 or the microsomal, mitochondrial, or bacterial Cytochrome P450s; these enzymes form almost entirely or partly 15"R"-hydroxy products. (15-Epi-LTA44 and 15-epi-LTB44 are sometimes termed AT-LxA4 and AT-LxB4, respectively, when acknowledging their formation by aspirin-treated cyclooxygenase 2, i.e. by Aspirin-Triggered cyclooxygenase 2.) In addition to the pathways cited above, other transcellular metabolic routes have been shown to make LXs. For example, 5-lipoxygenase (i.e. (ALOX5) in neutrophils and 15-lipoxygenase-1 (i.e. ALOX15) in immature erythrocytes and reticulocytes operate in series to form LxA44 and LxB44; this pathway also occurs in serial interactions between neutrophils and eosinophils; between epithelium or M2 Macrophages/monocytes and neutrophils; and endothelium or skeletal muscle and neutrophils. The lipoxins commonly form as a consequence of stimulating the production of pro-inflammatory arachidonic acid metabolites. However, certain cytokines such as IFN-γ and IL-1β further increase production of the lipoxins (as well as other anti-inflammatory PUFA metabolites and proteins, e.g. IL4. LXs are rapidly metabolized, mainly by macrophages, to inactive products by being oxidized at carbon 15 to form 15-keto (also termed 15-oxo) LX products by a 15-hydroxyprostaglandin dehydrogenase; 15-oxo-LXA4 may be further metabolized to 13,14-dihydro-LXA4 by an oxidoreductase. 15-Epi-LXA4 and 15-epi-LXB4 are more resistant to the dehydrogenation enzyme than their LX epimers. In consequence of the operation of this anabolic pathway, LXs have very short half-lives "in vivo", the epi-LXs have longer "in vivo" half-lives and thereby greater potencies than their LX epimers, and synthetic lipoxins that are metabolically resistant to this pathway have been prepared, used in animal models to study LX activities, and tested as potential therapeutic agents in animals and humans.