Georgia Tech / EllaMaru Studio (work for hire, all rights GT, free for distribution).
A T cell, here in purple, makes contact with a transplant organ cell, here in reddish brown. The T cell secretes the enzyme granzyme B, here in gray, which attacks the organ cell. But granzyme B also severs fluorescent signal molecules, in green, from the rejection detecting nanoparticle, in light pink. The signal molecules make their way into the urine, where they give off a fluorescent cue.