That protein may be responsible for itching, swelling and rashes suffered by people taking a wide range of medicines.
Such reactions stop people completing treatments and can sometimes be fatal.
Writing in the journal Nature, scientists say they are exploring ways to block the protein and reduce these side-effects.
Researchers from the Johns Hopkins University, in Maryland, and the University of Alberta focused on reactions triggered by medicines prescribed for a number of conditions - from diabetes to HIV.
These reactions, also seen after some antibiotics or anti-cancer treatments, can spark a range of symptoms from redness to rashes.
They are different to the allergic reactions caused by food and those experienced by hay fever sufferers.
Scientists tested mice with and without a single protein - named MRGPRB2 - on their cells.
Mice without the protein did not suffer any redness, rashes or swelling despite being given drugs known to provoke reactions.
And changes in blood pressure and heart rate - hallmarks of potentially dangerous reactions - were reduced.
Dr Benjamin McNeil, at John Hopkins University School of Medicine, said: "It's fortunate that all of the drugs turn out to trigger a single receptor - it makes that receptor an attractive drug target."
And if a new drug to block the protein receptor could be made, Dr McNeil said, this would help reduce the side-effects many patients currently endured.
Maureen Jenkins, clinical director of Allergy UK, said: "Allergic disease affects the immune system and the reactions are often very complex.
"All new methods to try and understand these reactions and to develop target treatments are welcomed."
Scientists are now investigating whether the same protein could be behind certain skin conditions - such as rosacea and psoriasis.
These conditions can result in patches of redness and rashes, but their cause is currently unknown.
Allergic reactions to drugs and injections could stem from one single protein, research in mice suggests.