We began the experiment with tail vein administra=on of fluorescein-labeled dextran to locate candidate imaging sites, followed by co-administra=on of the PE-labeled control an=bodies and α-TER119-iTCO-AZ647. Diffuse plasma signal from the free an=bodies was visible within seconds of the IV injec=on, which progressed to bright cell-specific labeling as an=body binding occurred on the =mescale of minutes (Supplementary Fig. ). Ini=al intravital images collected approx. 10 minutes aUer an=body injec=on revealed bright staining of circula=ng neutrophils in the PE reference channel and excellent RBC-specific signal in both the AZ647 and the PE channel. Background signal in the capillary bed was minimal, indis=nguishable from unstained extravascular =ssue (Fig ), consistent with near-complete an=body binding. We then administered an IV bolus of Pyr2(PEG24)3 to trigger the in vivo cleavage reac=on. At a weight-based dose to obtain a plasma Tz concentra=on of ~10 µM, our in vitro data (Fig. ) predicts the ini=al Pyr2/iTCO click reac=on to proceed with a t1/2 of ~7-14 seconds, se{ng a minimum expected =meframe for ini=a=on of release.