serum immunoglobulin measurement serum immunoglobulin M level Clinical_Measurement.ontology CMO:0002104 2014-01-27T12:35:25Z The amount in a specified sample of serum of one or more types of immunoglobulin M (IgM), one of the five classes of antibodies produced by the body. IgM is found in circulating body fluids and is the first class of antibodies to appear in response to an initial exposure to an antigen. JSmith serum IgM level