Immunoglobulin M (IgM) is a type of antibody produced by the immune system. It’s one of the first antibodies to be produced in response to an infection, particularly during the early stages of an immune response. IgM is mainly found in the bloodstream and lymphatic system. Testing the levels of IgM in the serum can provide valuable information about the body’s immune response to various infections or diseases. Elevated levels of IgM may indicate an acute or recent infection, while low levels could suggest immune deficiency or a chronic infection.