Scientists have discovered a specific protein that triggers allergic reactions when it comes into contact with fragrances common in cosmetic products.
It can send immune cells into overdrive when people use skin creams, body washes, lotions, shampoos - and even toothpastes.
The breakthrough could help stem soaring cases of rashes, lumps, blisters, itchy eyes and facial swellings. It has been dubbed the 'molecular missing link'.
An average woman uses 12 cosmetic items a day - containing an estimated 168 different chemicals.
And as these products become increasingly common, scientists have noted that contact dermatitis - a kind of allergic reaction that causes a red, itchy rash skin touches certain substances - has also been on the rise.
The key protein, called CD1a, is found in immune cells that form the outer layer of human skin. It offers hope of developing drugs that block it.
Experiments showed it binds directly to allergens, or irritants, found in personal care products - triggering the immune system's T cells.
These white blood cells kill foreign invaders. Allergies stem from mistaken identity, when they respond to benign substances that would otherwise be harmless.
Co senior author Professor David Branch Moody, of the division of rheumatology, inflammation and immunity at Brigham and Women's Hospital in the United States, explained: "What we present here is a molecular missing link."
Compounds found in skin creams and other cosmetics can cause ACD (allergic contact dermatitis), he said.
The common condition is on the rise, particularly in industrialized countries. But the exact reasons remain unclear.
Most allergies involving T cells are attributed to proteins or synthetically produced peptide antigens that set off the immune system.
But chemicals found in personal care products are different kinds of molecules that were not thought to be able to directly elicit a reaction.
The latest findings, published in the journal Science Immunology, uncover the way in which components of everyday consumer products do just this.
They shed light on the mystery behind the rise in cases of ACD, say the international team.
Prof Moody said: "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."
Dermatologists have been puzzled for years as to how allergies are triggered by many substances found in soaps, cosmetics, fragrances, jewellery and plants.
The molecules were thought to be too small and of the wrong chemical structure to be detected directly by T cells - the immune cells that fuel ACD.
Co study leader Dr Annemieke de Jong, a skin immunologist at Columbia University, New York, and colleagues wondered if there might be another explanation.
So first author Dr Sarah Nicolai, MD, a research fellow in medicine at Brigham, exposed T cells to material from skin patch testing kits used in allergy clinics.
This showed they responded to certain substances including balsam of Peru - widely used in cosmetics and toothpaste.
The researchers further identified ingredients within the tree oil - benzyl benzoate and benzyl cinnamate - that were directly responsible for stimulating the reaction.
They also tested similar substances and found a dozen small molecules that appeared to elicit a response.
These included farnesol - an alcohol used in a host of products including deodorants, anti ageing creams, shower gels and shampoos.
Using scanning techniques, further analysis by investigators at Monash University, Melbourne, revealed farnesol and CD1a combine to destroy naturally occurring blood fats. This makes the protein more visible to T cells - helping to activate them.
The researchers now plan to see if patients commonly have T cells that recognise molecules like farnesol.
They also seeking new molecules that could block the response of CD1a and override the activation of T cells. Work is currently underway to identify promising compounds.