Immunoglobulin M (IgM) is a basic antibody that is produced by B cells. It is the first antibody to appear in response to initial exposure to an antigen. The spleen, where plasmablasts responsible for antibody production reside, is the major site of specific IgM production. IgM in normal serum is often found to bind to specific antigens, even in the absence of prior immunization. For this reason IgM has sometimes been called a “natural antibody”. Demonstrating IgM antibodies in a patient’s serum indicates recent infection, or in a neonate’s serum indicates intrauterine infection.