Immunoglobulin M (IgM): Found mainly in blood and lymph fluid, this is the first antibody the body makes when it fights a new infection. IgM definition is - a class of immunoglobulins of high molecular weight including the primary antibodies that are released into the blood early in the immune. Beckman Coulter's chief medical officer, Dr. IgM is usually the first antibody produced by the immune system when a virus attacks. Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. Immunoglobulin M (IgM), which is found mainly in the blood and lymph fluid, is the first antibody to be made by the body to fight a new infection.