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. In humans and other mammals that have been studied, plasma blasts in the spleen are the main source of specific IgM production. IgM is the first immunoglobulin expressed in the human fetus (around 20 weeks) and phylogenetically the earliest antibody to develop. IgM antibodies appear early in the course of an infection and usually reappear, to a lesser extent, after further exposure. 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.