Immunoglobulin M is a pentamer found in the intravascular compartment and on the surface of B lymphocytes. It is the antibody isotype produced initially in the immune response, and the first immunoglobulin class to be synthesized by a fetus or newborn. IgM antibodies do not cross the placenta. Decreased levels of IgM have been associated with autoimmune disease, several primary immunodeficiency but exist also as selected primary immunodeficiency.