Non-Caseating Granulomatous Reaction to Red Tattoo Pigment in a Patient Receiving Immune Checkpoint Inhibitor Therapy.

Cutaneous adverse reactions associated with tattoos exhibit various histopathologic patterns, ranging from allergic or interface dermatitis to sarcoid-like granulomatous inflammation. Immune checkpoint inhibitors (ICIs) are also well known to induce granulomatous cutaneous eruptions, usually with sarcoid-like features. Interestingly, ICIs can enhance immune responses at previous antigen exposure sites, including tattoos. Herein, we report a case of a non-caseating granulomatous tattoo reaction associated with red pigment in the context of ICI therapy. Histopathologically, the granulomas exhibited a tuberculoid architecture characterized by prominent peripheral lymphoplasmacytic inflammation rather than the relatively "naked" granulomas more typical of sarcoidal reactions. This case highlights the morphologic heterogeneity of granulomatous reactions arising within tattoos during ICI therapy and underscores the complex interaction between host immunity, tattoo pigment, and immunotherapy. Awareness of this pattern may help prevent misclassification as cutaneous sarcoidosis or infection and guide clinical care for patients on ICIs who develop such granulomatous reactions.