Skin immune systems and inflammation: protector of the skin or promoter of aging?

The immune system may either have a protective role against sunburn and skin cancer or, conversely, promote solar damage. The skin is poised to react to infections and injury, such as sunburn, with rapidly acting mechanisms (innate immunity) that precede the development of acquired immunity and serve as an immediate defense system. Some of these mechanisms, including activation of defensins and complement, modify subsequent acquired immunity. An array of induced immune-regulatory and pro-inflammatory mediators is evident, at the gene expression level, from the microarray analysis of both intrinsically aged and photoaged skin. Thus, inflammatory mechanisms may accentuate the effect of UV radiation to amplify direct damaging effects on molecules and cells, including DNA, proteins, and lipids, which cause immunosuppression, cancer, and photoaging. A greater understanding of the cutaneous immune system's response to photo-skin interactions is essential to comprehensively protect the skin from adverse solar effects. Sunscreen product protection measured only as reduction in redness (current "sun" protection factor) may no longer be sufficient, as it is becoming clear that protection against UV-induced immune changes is of equal if not of greater importance. Greater knowledge of these processes will also enable the development of improved strategies to repair photodamaged skin.