Systems and methods apply antibody labeling to quantify perforin-positive lymphocytes per unit area in an allograft biopsy, for detection and characterization of transplant rejection. An example technique includes staining the allograft biopsy to visualize lymphocytes, applying labeled anti-perforin antibodies to the allograft biopsy, visualizing the labeled anti-perforin antibodies to show perforin-positive lymphocytes, quantifying the perforin-positive lymphocytes to determine a count of the perforin-positive lymphocytes per unit area, and classifying a rejection parameter of the allograft based on the count. The example technique can be used to stratify patients into distinct risk groups with regard to presence of and type of allograft rejection. An example imaging system introduces a labeled anti-perforin antibody into a transplanted tissue in vivo, obtains images of the in vivo labeled anti-perforin antibody in the transplanted tissue, and determines a presence or a degree of tissue rejection based on the images.
