Gene-targeting therapies could one day offer relief from allergies such as hayfever, say UK and Swiss scientists.
The excretion of the house dust mite can cause allergy
A gene called "GATA-3" can stop the body's immune system from working properly when it meets potential allergens, they say.
The journal PLoS Biology reports that mice with dominant GATA-3 didn't produce enough key immune cells to prevent allergy attacks.
A treatment may still be some years away, allergy specialists have warned.
In most people, coming into contact with pollen, animal hair or nuts causes no reaction because their immune systems recognise them as harmless.
However, in allergy sufferers, the immune system becomes programmed to see them as a threat, launching an attack which causes inflammation, wheezing or rashes.
Scientists have spent many years looking at why and how the body responds this way.
One of the most important discoveries was the "regulatory T-cell", which appears to have some beneficial control on the scale and direction of unwanted immune system attacks.
The latest find, from Imperial College London and the Swiss Institute of Allergy and Asthma Research in Davos, has revealed more about the genes important to the production of regulatory T-cells.
Activity in two different genes appears to be crucial - the FOXP3 gene which helps make the cells, and the GATA-3 gene, which, when over-active, blocks FOXP3.
They used mice engineered to "over-express" GATA-3 to test this theory, and found that regulatory T-cell numbers were much lower.
Dr Carsten Schmidt-Weber, the lead investigator on the research from Imperial's National Heart and Lung Institute, said: "This finding will help us to understand how healthy individuals are able to tolerate allergens and what we need to do to reinduce tolerance in the immune systems of patients with allergies."
Although allergy is known to be inherited in many cases - suggesting that genes are involved - the huge rise in the number and severity of allergies reported to doctors in recent decades is also thought to be due to other, environmental factors.
Some estimates suggest that the number of people in the UK with some form of allergy is at least 18 million.
A spokesman for the charity Allergy UK warned that the latest gene find would not lead immediately to useful treatments.
"All research is useful and interesting but a cure for allergy and indeed the sort of treatment that could direct the T-cells to avoid allergy is many years down the line."