Immediate pigment-darkening reaction.

The immediate pigment-darkening reaction (IPD) is a gray-brown discoloration of the skin induced mainly by long-wave ultraviolet (UVA) radiation. The threshold value of IPD can be used as an important parameter in a series of light tests. The principle mechanism of the reaction is a photo-oxidative process causing darkening of preformed melanin and the production of free radicals. It has also been suggested that the reaction involves micromorphological changes in the melanocytes and a changed distribution pattern of melanosomes in the keratinocytes. This concept has recently been questioned by independent investigators. The physiological function of IPD still remains largely unknown. No photoprotective effect of the reaction has been observed. It has been suggested that IPD might have been of value during human evolution.