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IgM immunoglobulins are produced by plasma cells as part of the body's adaptive humoral immune response against a foreign pathogen. Resting mature yet naive, B lymphocytes express IgM as a transmembrane antigen receptor that functions as part of the B-cell receptor (BCR). B cell activation in response to antigen binding to the BCR results in rapid cell division and clonal expansion of the activated B lymphocyte, producing many progeny cells that can differentiate into antibody-secreting plasma cells or memory B lymphocytes. Antibodies are similar in structure to transmembrane immunoglobulins yet lack a short transmembrane segment at the carboxy-terminal end. IgM is the first antibody secreted by the adaptive immune system in response to a foreign antigen. Monomeric IgM is a heterotetramer of approximately 180 kDa. However, the secreted form of IgM exists predominantly in a pentameric configuration with a molecular weight greater than 900 kDa. In serum, the IgM has a half-life of approximately 5 to 10 days and is composed of approximately 12% carbohydrates by weight.[1] The average serum concentration of IgM in the body is about 1.5 mg/ml.