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Understanding allergic reactions to skin care products

Researchers have been working to understand how chemical compounds in personal care products can trigger rashes. zoranm / E+ via Getty Images

Skin breaks out in an itchy red rash when exposed to something that causes an allergic reaction, like poison ivy. Personal care products like makeup, skin cream, and fragrances also commonly cause rashes. It’s not well understood how chemical compounds in personal care products trigger these rashes, called allergic contact dermatitis.

Many allergic reactions start when immune system cells known as T cells detect a foreign substance, called an antigen, and attempt to neutralize it. A common mechanism for allergic reactions involves T cells recognizing parts of proteins, or peptides. However, personal care products contain other types of compounds that were believed to go undetected by T cells. They were thought to be too small and lacking the necessary chemical groups.

Researchers set out to uncover how such chemical compounds in personal care products could trigger a T cell reaction. The research team was led by Drs. Annemieke de Jong of Columbia University, D. Branch Moody of Brigham and Women’s Hospital and Harvard Medical School, and Jamie Rossjohn of Monash University and Cardiff University School of Medicine.

The research was funded in part by NIH’s National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). Findings appeared in Science Immunology on January 3, 2020.

The researchers suspected that a protein called CD1a, found on the surface of immune cells in the skin’s outer layer, might play an important role in explaining how personal care products could trigger contact dermatitis. To investigate the molecule’s role, they exposed human cells in tissue culture to potential allergens in personal care products. This was done using skin patch testing kits used by allergists. The allergen patches were cut and placed directly into wells containing T cells. The researchers then measured T cell activation.

The research team found that several chemicals caused an immune reaction, including balsam of Peru, a tree oil often used in cosmetics and toothpaste. Further analysis of balsam of Peru revealed that the chemicals benzyl benzoate and benzyl cinnamate were responsible for stimulating the T cell response.

The scientists found that the compound farnesol, often used as a fragrance in personal care products, also triggered an allergic reaction. Using X-ray crystallography, they were able to determine how CD1a binds with compounds like farnesol to elicit an immune response.

Farnesol buries deep within CD1a’s tunnel-like interior, displacing the larger lipid molecules usually found there. These lipids protrude onto the surface of the CD1a protein. Without them, parts of the molecule that are normally hidden interact with T cells and trigger an immune response.

These findings explain how common chemicals in personal care products can directly trigger T cells. The next step will be to confirm whether these T cells cause allergic contact dermatitis in patients.

“What we present here is a molecular missing link,” says Moody. “We questioned the prevailing paradigm that T cell-mediated allergic reaction is only triggered when T cells respond to proteins or peptide antigens. We find a mechanism through which fragrance can initiate a T cell response through a protein called CD1a.”

—by Erin Bryant