Watch this video of photodynamic therapy.
Left Panel: In untargeted PDT, photosensitizers (yellow circles) can accumulate in diseased as well as healthy tissues. Following light treatment, the photosensitizers can cause undesired off-target damage.
Right Panel: In targeted PDT, photosensitizers (orange circles) are stably attached to biomolecules that specifically recognize diseased tissue. The biomolecules enable the photosensitizers to accumulate in diseased tissues and avoid healthy tissues, thereby reducing undesired off-target damage following light treatment. http://bit.ly/1KaPuz0