IgM (Immunoglobulin M)

IgM (Immunoglobulin M) is the first antibody class produced by the immune system when it encounters a new infection or antigen. This test measures the level of IgM in a simple blood sample, using an immunoassay technique to detect and quantify the antibody. It is used to investigate suspected recent or acute infection, to assess immune function, and to support the diagnosis of certain autoimmune and haematological conditions.

IgM is the largest immunoglobulin, forming a pentamer that is highly effective at activating complement and clumping pathogens early in the immune response. Because it appears before IgG, a raised IgM level often indicates a current or recent infection, while low levels may point to an impaired immune response or a rare immunodeficiency.

What this test screens for

• Acute or recent infection: Raised IgM can indicate a current or recent infection, including viral, bacterial, and some parasitic infections.
• Autoimmune conditions: IgM levels may be altered in conditions such as rheumatoid arthritis, systemic lupus erythematosus, and primary biliary cholangitis.
• Immunodeficiency: Low or absent IgM can suggest an impaired humoral immune response, including rare primary immunodeficiencies.
• Haematological disorders: Abnormal IgM can be associated with conditions such as Waldenström macroglobulinaemia and other monoclonal gammopathies.

Why it matters

IgM is the earliest antibody your body makes in response to a new threat, so measuring it gives clinicians a valuable window into whether an infection is recent or ongoing, and whether the immune system is responding appropriately. It also helps identify autoimmune and haematological conditions that can be managed more effectively when detected early.