Immunoglobulin IgM are antibodies found in the blood and lymph fluid that play a key role in the body's initial immune response to infections. They are produced early in the course of an infection to help fight off harmful pathogens. The IgM test measures the level of these antibodies in the blood.
A low level of IgM can indicate that your immune system is weakened or not functioning properly, which may leave you more vulnerable to infections.