There are several factors that can cause your underarms to break out in a rash. Three of the most common culprits are heat, friction and contact with allergens.
Heat rash occurs when your sweat ducts are blocked and perspiration builds up beneath the skin, causing a bumpy and sometimes painful inflammation. It's not always clear why sweat ducts become blocked, but certain factors can contribute to it, including bacteria, tight-fitting clothing, medications and certain fabrics [source: Mayo Clinic].
Friction can also cause skin irritation in the underarms -- this is an area of the body where skin continually rubs against fabric or other skin. Friction, combined with heat and moisture, breaks down the epidermis and can cause inflammation. While this may be uncomfortable enough on its own, it's often complicated when bacteria or fungus colonize among the broken skin and infect it [source: Selden]. One specific type of infection that may develop in this manner is candidiasis, or a yeast infection [source: Ripper].
Contact dermatitis is a very common cause of skin irritation on the body, including the underarms. Contact dermatitis works just like it sounds -- if something touches your skin that irritates it or causes a reaction, a rash can develop. When this occurs in the underarms, deodorant or antiperspirant is often the culprit. However, if the rash is bumpy or blistering, an allergy may be responsible [source: Mayo Clinic].
Keep reading to learn how to treat and prevent underarm rashes.