Allergic reactions in the skin can be caused by many different chemical
compounds found in creams, cosmetics, and other topical consumer products, but
how they trigger the reaction has remained somewhat mysterious.
A new study
suggests the way some chemicals displace natural fat-like molecules (called
lipids) in skin cells may explain how many common ingredients trigger allergic
contact dermatitis, and encouragingly, suggests a new way to treat the
condition.
The study was led by researchers at Columbia University Irving Medical
Center, the Brigham and Women’s Hospital, and Monash University and published
online today in Science Immunology.
Why some chemicals trigger
dermatitis is a mystery
Poison ivy is a
commonly known trigger for allergic contact dermatitis, an itchy skin rash. But
many ingredients found in nonprescription topical products can trigger a
similar type of rash.
An allergic
reaction begins when the immune system’s T cells recognize a chemical as
foreign. T cells do not directly recognize 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,” says study co-leader Annemieke de Jong, PhD, assistant
professor of dermatology at Columbia University Vagelos College of Physicians
and Surgeons.
“These small
chemicals should be invisible to T cells, but they’re not.”
Skin
cells unmask allergy-inducing chemicals
De Jong and her
colleagues suspected that CD1a, a molecule that’s 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.
“Our work shows
how these chemicals can activate T cells in tissue culture, but we have to be
cautious about claiming that this is definitively how it works in allergic
patients,” de Jong says. “The study does pave the way for follow up studies to
confirm the mechanism in allergic patients and design inhibitors of the
response.”
New Ideas for Treatment
CD1a molecules
normally bind the skin’s own naturally occurring lipids in its tunnel-like
interior. These lipids protrude from the tunnel, creating a physical barrier
that prevents CD1a from interacting with T cells.
Structural work
done at Monash University showed that farnesol, one of the allergens identified
in this study, can hide inside the tunnel of CD1a, displacing the natural
lipids that normally protrude from the CD1a molecule. “This displacement makes
the CD1a surface visible to the T cells, causing an immune reaction,” de Jong
says.
This discovery
raises the possibility that allergic contact dermatitis could be stopped by
applying competing lipids to the skin to displace those triggering the immune
reaction. “From previous studies, we know the identity of several lipids that
can bind to CD1a but won’t activate T cells,” she says.
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.
Source: https://www.cuimc.columbia.edu/news/study-explains-why-some-creams-and-cosmetics-may-cause-skin-rash
Journal article: https://immunology.sciencemag.org/content/5/43/eaax5430
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