Caffeine allergy, also called an IgE-mediated hypersensitivity reaction to caffeine, is very rare.
Only a handful of cases of caffeine allergy have been reported within and outside of the United States. The known symptoms of caffeine allergy are hives (urticarial) and anaphylaxis reactions. (1, 2, 3, 4)
Anaphylaxis reaction usually occurs due to the combination of caffeine with other drugs (e.g. aspirin) rather than caffeine on its own, although there have been some cases of anaphylaxis reactions reported outside of the United States from caffeine alone. (5)
Considering that 85% of the U.S. population drinks at least one caffeinated drink per day (6), the allergy to caffeine is considered extremely rare.