Immunoglobulin M: Your body makes IgM antibodies when you are first infected with new bacteria or other germs. They are your body's first line of defense against infections. When your body senses an invader, your IgM level will rise for a short time. It will then begin to drop as your IgG level kicks in and increases to protect you long-term.

Immunoglobulin M (IgM) is the largest of several isotypes of antibodies (also known as immunoglobulin) that are produced by vertebrates. IgM is the first antibody to appear in the response to initial exposure to an antigen; causing it to also be called an acute phase antibody.