Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen. IgM is the first immunoglobulin expressed in the human fetus (around 20 weeks) and phylogenetically the earliest antibody to develop. IgM antibodies do not pass across the human placenta (only isotype IgG). Demonstrating IgM antibodies in a patient's serum indicates recent infection, or in a neonate's serum indicates intrauterine infection (e.g. congenital rubella syndrome). IgM in normal serum is often found to bind to specific antigens, even in the absence of prior immunization. A mutation of the mu chain within IgM causes autosomal recessive agammaglobulinemia. The presence of IgM or, rarely, IgG is one of the obligate criteria for a diagnosis of Schnitzler's syndrome.