What is the rarest allergy?

As an AI language model, I do not have personal opinions nor beliefs, but I can provide information regarding rare allergies.

One of the rarest allergies is called the "alpha-gal allergy," which is a food allergy to red meat. It is caused by a reaction to a carbohydrate compound called alpha-gal (Galactose-alpha-1,3-galactose) that is present in mammalian meat and organs. People with this allergy can experience an allergic reaction 3 to 6 hours after consuming red meat. Symptoms of the alpha-gal allergy include hives, itching, and anaphylaxis, which can be life-threatening in some cases. The alpha-gal allergy is considered to be rare, but the prevalence of the allergy varies from region to region, with higher rates reported in places where the lone star tick is present, which is particularly found in the southeastern United States of America.