Why some cosmetics may cause a skin rash

Do you know why many different chemical compounds found in creams, cosmetics, and other topical consumer products trigger allergic reactions in the skin? The way some chemicals displace natural fat-like molecules -- called lipids -- in skin cells may be the culprit, suggests new research.

This discovery, published in the journal Science Immunology, raises the possibility that allergic contact dermatitis could be stopped by applying competing lipids to the skin to displace those triggering the immune reaction.

Currently, the only way to stop allergic contact dermatitis is to identify and avoid contact with the offending chemical. Topical ointments can help sooth the rashes, which usually clear up in less than a month.

In severe cases, physicians may prescribe oral corticosteroids, anti-inflammatory agents that suppress the immune system, increasing the risk of infections and other side effects.

An allergic reaction begins when the immune system's T cells recognise a chemical as foreign. T cells do not directly recognise small chemicals, and research suggests that these compounds need to undergo a chemical reaction with larger proteins in order to make themselves visible to T cells.

"However, many small compounds in skincare products that trigger allergic contact dermatitis lack the chemical groups needed for this reaction to occur," said study co-leader Annemieke de Jong, Assistant Professor of dermatology at Columbia University Vagelos College of Physicians and Surgeons in New York.

"These small chemicals should be invisible to T cells, but they're not," De Jong said.

De Jong and her colleagues suspected that CD1a, a molecule that is abundant on Langerhans cells (immune cells in the skin's outer layer), might be responsible for making these chemicals visible to T cells.

In the current study, conducted with human cells in tissue culture, the researchers found that several common chemicals known to trigger allergic contact dermatitis were able to bind to CD1a molecules on the surface of Langerhans cells and activate T cells.

These chemicals included Balsam of Peru and farnesol, which are found in many personal care products, such as skin creams, toothpaste, and fragrances.

Within Balsam of Peru, the researchers identified benzyl benzoate and benzyl cinnamate as the chemicals responsible for the reaction, and overall they identified more than a dozen small chemicals that activated T cells through CD1a.