Two-photon microscopy has shown that skin absorbs various substances differently, depending upon what is mixed with them. These differences may determine whether a material causes contact allergy. "We have also been able to identify specific cells and proteins in the skin with which a contact allergen interacts," said Carl Simonsson of the University of Gothenburg. "The results increase our understanding of the mechanisms behind contact allergy." The skin is the largest organ in the human body and plays many vital roles, one of which is to prevent harmful microorganisms from invading the body. The principal barrier is the stratum corneum — a layer of skin cells about a few microns thick. Despite being so thin, this layer effectively protects us from bacteria and viruses. Skin photographed in a two-photon microscope, showing epidermal cells and the collagen present in the dermis. Courtesy of Carl Simonsson. The skin, however, has not adapted to deal with and prevent absorption of many of the chemicals to which we are exposed today. This may lead to various types of diseases, such as contact allergy, which affects approximately 20 percent of people in Sweden. With two-photon microscopy, substances can be followed as they are absorbed into the skin. The method is unique in that it allows researchers not only to see how well a substance is absorbed, but also what happens to it, and to find the location in the skin where the substance eventually arrives. Carl Simonsson, whose thesis showed the utility of two-photon microscopy in the exploration of contact allergens in the skin. Courtesy of University of Gothenburg. In addition, the skin barrier and the way in which various substances are absorbed are highly significant for the development of new drugs. Creams and ointments are for many reasons an interesting alternative to tablets, which must be taken orally. The barrier properties of the skin may present, in this case, an obstacle to drug absorption, making it difficult for sufficient amounts of the drug to penetrate the skin to produce a clinical effect. "We have used two-photon microscopy to study a new type of ointment that it may be possible to use to improve the absorption, and thus the clinical effect, of certain drugs that are used on the skin," Simonsson said.