Immunoglobulin E is a form of immunoglobulin antibody involved in the immune system's response to parasites and cancer. However, it also plays a role in allergies, particularly those that cause asthma. It is the least common of the immunoglobulin types, making up less than 1/2 of 1% of the body's immunoglobulin. An excess of immunoglobulin E is a key finding in hyper IgE syndrome.