In an ARDS model, CD45-PET showed superior performance over a CD11b-targeted probe and uniquely resolved high- versus low-dose LPS groups, whereas CD11b-PET did not.1 Mechanistically, the favorable performance is best explained by the high and stable antigen density on leukocytes, although CD45 is largely non-internalizing and preserves surface accessibility during the imaging window, receptor internalization can, under certain tracer-target kinetic conditions, increase cellular trapping and retention. Here, PTPRC is linked to acute respiratory distress syndrome.