An allergic reaction also referred to as a hypersensitivity reaction is an overactive or misdirected immune response that results in local tissue injury or changes throughout the body in response to a foreign substance. These reactions can be caused by many factors, including chemo treatments. Our body's immune response to a foreign substance is potentially a "two-edged sword" it can either protect or harm the host. It protects us from foreign substances such as viruses or bacteria, but it can also bring about an exaggerated (hypersensitive) response when previously sensitized individuals are exposed to the same foreign substance.