They said they had successfully prevented even acute allergic responses such as anaphylactic shock, which can kill.
Scientists had previously discovered that coffee cold have anti-allergic properties because of its ability to reduce the release of histamine from mast cells.
In susceptible people, mast cells release histamine into affected tissues when triggered by substances such as pollen and dust.
The researchers at Wongkwang University in South Korea are the first to investigate its role in stopping anaphylactic shock.
They injected rats with a synthetic compound called 48/80 to activate mast cells and rapidly release large amounts of histamine.
In untreated rats, the injections caused fatal allergic reactions, but when they repeated the test giving the rats an infusion of caffeine, many survived.
Levels of caffeine as low as 0.1 milligrams per kilogram - equivalent to a strong cup of coffee - cut the death rate by half, reported New Scientist magazine.
Hopeful
Hyung-Min Kim, who led the research, was hopeful caffeine could have a similar effect in humans.
He said: "Caffeine may be effective in chronic allergic disorders."
And it might be useful in preventing everyday allergies, he added.
Francesca Levi-Schaffer, a pharmacologist at the Hebrew University-Hadassah Medical School in Israel, said: "Caffeine can be a preventative drug if given by inhalation to asthmatic patients."
A similar substance called theophylline had a similar effect in inhibiting the enzyme involved in allergic reactions and was more powerful.
"Caffeine works to an extent, but I would expect it to have a small effect, not a dramatic effect."
He added: "The levels required to inhibit reaction would be too high to be achieved by drinking a cup of tea or coffee."
Other drugs existed which were more successful in preventing allergic reactions and they did not have the side effects that would be associated with caffeine, he added.
Dr Pam Ewan, president of the British Society for Allergy and Clinical Immunology, said: "It all sounds a bit far-fetched to me. It is hard to think of a mechanism whereby it would work."