Adenosine, synthesized continually de novo in human and animal cells, is of fundamental importance not only as a nucleic acid (DNA, RNA) building block, but also as the backbone for the energy storage‐and‐use system of living cells (ATP) and as a regulator of biologic function, modulating adrenergic stimulation and inflammation. These latter effects are critical to the body’s natural defense against tissue oxygen‐deprivation damage or ischemia, leading to adenosine’s designation as the “retaliatory metabolite.”