The seed is surrounded by a double shell containing an allergenic phenolic resin containing a anacardic acid. This composition is a potent skin irritant. Chemically, this resin is related to the more well known allergenic, oil urushiol, which is also a toxin found in poison ivy. Some people are allergic to cashew nuts. What is much more common is the allergy to outer coating on the skin of the fruit.

The cashew was first discovered by Europeans in Brazil around 1558. Because of the irritating shells, they were thought to be inedible at first. Over time it was realized that it was the fruit skin and not the seeds that were irritating. It was the local native tribe, the Tupi-Indians, that showed the Europeans otherwise. The Portuguese were given instructions on how to roast the cashews to get the irritant off.

Europeans found the cashew seed to be quite tasty. However, they also used the cashew apple pulp to make wine. Interestingly enough, the natives had learned to eat cashews from the local capuchin monkeys. The primates use primitive tools to break the shells off and throw the nuts in away.