It's certainly not the sexiest topic, but armpit rash and armpit irritation are very real things—very real, very irksome, and very uncomfortable things, at that. "The skin in this area is thinner compared to most parts of the body, making it more sensitive and susceptible to environmental irritants," explains board-certified dermatologist Teresa Song, MD. All these factors make this spot more prone to irritation (or intertrigo, a rash or inflammation in a skin fold).