Although reactive large granular lymphocytosis due to diverse etiologies is not an uncommon finding in clinical practice, isolated NK-cell lymphocytosis is unusual and its association with immunotherapy has not been described thus far. We provide a brief analysis of a patient with this unique hematological corollary of immunotherapy being increasingly used in the setting of both solid organ and hematological malignancies, and highlight this as an additional differential to consider in the diagnosis of large granular lymphocytosis. Also explored is the importance of recognising and monitoring the potential hematological manifestations of experimental immunotherapies, as well as the possible implicated mechanisms of action. It is hypothesized that quantification of large granular lymphocytosis may potentially be used as a surrogate marker of therapeutic efficacy in this setting.

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