One of the main functions of the liver is the removal of toxic substances from the blood. When the liver is diseased, a toxin called phylloerythrin increases in the bloodstream. Phylloerythrin is produced by the breakdown of chlorophyll (green pigment) present in plants eaten by the horse, and it is sensitive to light. When phylloerythrin reaches the skin and is exposed to ultraviolet sunlight, it releases energy and damages the skin. Unpigmented or light-skinned areas absorb the most ultra-violet light, so they are most likely to be affected by light sensitivity (photosensitization).