What is underarm rash? Rash is a symptom that causes the affected area of skin to turn red and blotchy and to swell. A rash may cause spots that are bumpy, scaly, flaky, or filled with pus. Rashes can vary in location, pattern and extent and may occur in any area of the body. A rash under your arm can have a variety of causes, and it may indicate something occurring around the underarm itself or suggest a systemic (body-wide) condition. Contact dermatitis (skin inflammation) is caused by an adverse reaction to something that touches your skin, including chemicals found in a detergent, soap, or fragrance. You may develop a rash under your arm when wearing a shirt that was washed with a particular detergent or treated with a chemical. Metal, such as jewelry, can also cause underarm rash. Other forms of contact dermatitis include exposure to certain plants, such as poison oak or ivy, an animal bite, or an insect sting. Lyme disease is caused by tick bite, which can first display as a circle with a bull’s-eye pattern, then progress to a rash. Rocky Mountain spotted fever is another tick-borne condition that may start with a rash on the arms and legs, followed by a fever and other flu-like symptoms. A rash under the arm can also be attributed to an infection from an ingrown hair from shaving. Allergies to food and medications are potentially serious sources of rash. Peanuts, shellfish, strawberries and avocados are just some of the foods that can trigger allergic reactions. These foods may cause mild reactions; however, in some cases reactions could develop into potentially life-threatening conditions characterized by vomiting, difficulty breathing, and swelling. Allergic purpura is a serious, often life-threatening allergic reaction that can cause a skin rash but can also affect the joints, gastrointestinal tract, and kidneys. Rashes may also be associated with skin conditions such as eczema, psoriasis and impetigo. These are skin conditions that may flare up for a time, then resolve. Other causes for rash include autoimmune disorders that occur when the body is attacked by its own immune system, which normally serves to protect it from foreign invaders (antigens). Many viruses that occur during flu season, or those associated with childhood diseases, can produce rash. Rashes can be caused by an allergic reaction to food, medications, or products such as lotions or detergents. These reactions can range from mild to potentially life threatening, especially if swelling and constriction of breathing occurs, which could indicate anaphylaxis. Seek immediate medical care (call 911) if a rash is accompanied by any serious symptoms, including swelling of the face, swelling or constriction of the throat, difficulty breathing, fainting, change in level of consciousness or alertness, or purple rash. Seek prompt medical care if a rash is persistent or causes you concern.