Immune cells travel all over the body, prowling for foreign agents to attack — including transplanted organs. The specifics of transplant rejection remain a topic of intense scientific scrutiny, but it’s been difficult for researchers to observe how and when immune cells gather at the organ during rejection, and how they act once they arrive.

Now, Alejandro Caicedo and his colleagues at the University of Miami Miller School of Medicine describe a new technique that provides a fresh perspective on the immune reaction triggered by organ transplants. In a study published online today in the Proceedings of the National Academy of the Sciences, Caicedo’s team transplanted mouse pancreatic cells into the eyes of living mice, and then waited for organ rejection. Using various cell dyes, they took advantage of the eye’s semi-translucence to watch the immune response happen in real time.

The eye “is a window into the body,” says Caicedo. “It’s easy to access and very easy to see.”

In the video below, you can see the results of their microscopy. The green blobs show the mouse T-cells, and the red demarcates the entire pancreatic cell transplant. Using this approach that is “unique to [their] method,” they were “able to follow the very same piece of tissue over time and repeatedly,” says Miller School postdoc and the study’s first author Midhat Abdulreda.

“These guys are not moving from one region to another, but their behavior is very frantic,” says Caicedo. “It’s very different from what other studies have shown.”

While this method puts the organ into a physiological context, a prominent scientist in the field who asked to remain anonymous doubts that the blood-vessel rich eye would replicate conditions in the pancreas, where auto-immune tissue rejection is one of the primary causes of type 1 diabetes. But the study authors disagree. Citing unpublished experiments, Abdulreda stresses: “We do believe that the immune response we’re seeing in the eye is a typical immune response.”