Buoyant mass reveals distinct T cell states predictive of checkpoint response

T cells are central to immune defense, yet existing molecular and phenotypic assays do not fully capture a cells intrinsic immune potential. Here we show that a single physical property, buoyant mass, reveals hidden heterogeneity within phenotypically similar, resting CD8+ T cells. Using suspended microchannel resonator measurements, we identify two distinct populations: "light" cells, enriched for mitochondrial content but prone to delayed activation and exhaustion, and "heavy" cells, biosynthetically poised for proliferation and memory formation. In patients with melanoma receiving immune checkpoint blockade, pre-treatment buoyant mass profiling of circulating T cells predicted therapeutic response with an accuracy comparable with standard tumor-derived biomarkers. Our findings establish buoyant mass as a label-free, stimulation-independent measure of systemic T cell fitness, providing a rapid and broadly applicable framework for immune profiling and response prediction in cancer and beyond.