['With increased access to antiretroviral treatment (ART), immune reconstitution inflammatory syndrome (IRIS) in Mycobacterium tuberculosis (MTB)-infected populations remains a clinical challenge. We studied a cross-sectional cohort of HIV-infected subjects in Johannesburg (South Africa) to help define the immune correlates that best distinguish IRIS from ongoing MTB cases.', 'We studied HIV+ subjects developing MTB-related unmasking tuberculosis-related immune reconstitution inflammatory syndrome (uTB-IRIS) after ART initiation; control groups were subjects with HIV and HIV/tuberculosis-coinfected subjects with comparable ART treatment. Testing was conducted with whole blood-based 4-color flow cytometry and plasma-based Luminex cytokine assessment.', 'Natural killer cell activation, C-reactive protein, and interleukin 8 serum concentration were significantly higher in uTB-IRIS subjects compared with both control groups. In addition, all MTB-coinfected subjects, independent of clinical presentation, had higher neutrophils and T-cell activation, together with lower lymphocytes, CD4⁺ T-cell, and myeloid dendritic cell counts. Using conditional inference tree analysis, we show that elevated natural killer cell activation in combination with lymphocyte count characterizes the immunological profile of uTB-IRIS.'] Does natural killer cell activation distinguish Mycobacterium tuberculosis-mediated immune reconstitution syndrome from chronic HIV and HIV/MTB coinfection?