tissue tropism, has a positive effect on the immune system and cell metabolism. This issue is subject to skepticism. Transcutaneous laser blood irradiation Transcutaneous therapy applies laser light on unbroken skin in areas with large numbers of blood vessels (such as the forearm). Because of the skin acting as a barrier to the blood, absorbing low level laser energy, the power of the laser is often boosted to compensate. The problem can be solved by using pulsed matrix laser light sources. Extracorporeal irradiation Extracorporeal irradiation is used only for ultraviolet blood irradiation, that involves drawing blood out through a vein and irradiating it outside of the body. Though promoted as a treatment for cancer, a 1952 review in the Journal of the American Medical Association and another review by the American Cancer Society in 1970 concluded the treatment was ineffective. See also Light therapy Photodynamic therapy Photopheresis References Light therapy Laser medicine Alternative medical treatments