What is the rarest allergy in the world?

The rarest allergy in the world is believed to be aquagenic urticaria, also known as water allergy. This extremely rare condition causes individuals to break out in hives or welts after coming into contact with water, regardless of its temperature. Symptoms can range from mild itching to severe rashes, and can occur after exposure to any form of water, such as bathing, swimming, or even sweating.

It is not fully understood what causes aquagenic urticaria, but it is believed to be an immune system reaction to certain components in water. Treatment options for this condition are limited, and individuals with aquagenic urticaria may have to take precautions to minimize their exposure to water.

Due to the extreme rarity of aquagenic urticaria, there are very few documented cases worldwide. This makes it one of the most unusual and least understood allergies in the medical field.