Downregulation of the immune response is common among Intensive Care Unit (ICU) patients after an acute inflammatory injury, whether it was septic or not. Such a modification could be seen as an adaptation to attenuate the effects of the inflammatory storm on tissues, but it exposes the subject to the risk of nosocomial infection and impairs recovery processes. The intensity of immunity downregulation is difficult to characterize, since clinical presentation is silent and non-specific, which urges the use of tools for immune monitoring. This review focuses on the use of monocyte HLA-DR expression to detect immune hyporesponsiveness and to select the appropriate immunomodulating therapy, as well as the efficiency of this technique in controlling secondary infections.