IgM (immunoglobulin M) is the body’s first response to a “foreign” antigen. It is produced during initial exposure to the antigen (bacteria, viruses, etc.), and it increases within a few weeks before being replaced by IgG. IgM levels are interpreted in light of IgA and IgG, the other two main classes of antibodies.
Low levels of IgM can, in rare cases, be due to genetic causes affecting the production of one or all classes of antibodies, but usually they are secondary to difficulties in the production of all types of immunoglobulins (immunosuppressive drugs, diabetes complications, kidney failure, etc.) or to protein loss through the kidneys, intestines or skin. Uncontrolled production of another class of immunoglobulin such as IgA or IgG (multiple myeloma) often causes a marked decline in the other classes.
High IgM levels at the same time as high IgG and IgA levels are seen in acute and chronic infections, autoimmune diseases (rheumatoid arthritis, lupus), cirrhosis of the liver, etc. Isolated (monoclonal) increases in IgM can be of undetermined cause (MGUS) or associated with multiple myeloma, or chronic lymphocytic leukemia or lymphoma. High levels of IgM (Waldenstrom macroglobulinemia) cause symptomatic thickening of the blood, especially in cold weather.