S100A9 and acquired immunodeficiency: As sole use of mHLA-DR expression might render limitations under certain circumstances, Venet et al. [48] proposed a novel yet universal immune monitoring panel incorporating mHLA-DR expression measured by flow cytometry and messenger RNA levels of S100A9, CD3 as well as CD74 for the prediction of acquired immunodeficiency among critically ill patients regardless of etiologies, which revealed encouraging results.