Critical illness may often induce persisting injury-associated immunosuppression with adverse effects on relevant patient-centered outcomes. However, despite the key task of ICU physicians to detect, monitor, and follow up on organ dysfunctions, functional failure of the “immune organ” seems currently mostly overlooked as it cannot be adequately assessed via use of routine biomarkers such as numerical distribution of leukocyte (sub)counts or systemic levels of soluble markers such as cytokines, procalcitonin, or acute phase proteins.

Importantly, quantitative assessment of a given cell population does not per se allow to conclude on its functional status.

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