All about perspiration > Why are my underarms red or bumpy?

Your underarm is 1% sweat glands and 99% skin. And because underarm skin is very thin and collects fluid quickly, it is even more prone to irritation than many other parts of our bodies. Moreover, the very things we do to care for it—frequent shaving among them—can dry and irritate it. This, plus harsh deodorants and skin-on-skin and clothes-on-skin chafing, leaves many women with irritated underarm skin.