A rash is a general term meaning an outbreak of bumps on the body that changes the way the skin looks and feels. Rashes can be localized to one area or be widespread over the body. The way people use this term, a rash can refer to many different types of skin condition. Hives are a very common form associated with allergies.
The most challenging types of allergies and skin rashes to diagnose are those caused by something ingested or something commonly found in the environment.
There are two types of rash: dermal, which is caused by touching a substance you are allergic to - and systemic, which is when the offending substance is ingested. Even allergies such as hay fever can cause a rash to appear. Sometimes a rash can develop for no apparent reason, only to discover later that you’ve suddenly become allergic to something you’ve eaten or used without trouble for years.