IGM IgM (immunoglobulin M) is the first antibody formed by a newly sensitized B-lymphocyte plasma cell. IgA has very low circulating levels and is responsible for preventing infection in the upper and lower respiratory tracts, and the gastrointestinal and genitourinary tracts. IgE has variable concentrations in the blood and is associated with antibody-mediated hypersensitivity reactions. IgG is heavily expressed on second and subsequent exposures to antigens to provide sustained, long-term immunity against invading microorganisms.