This test is your immune system's 'first responder' check. The Immunoglobulin M (IgM) test measures the total amount of the large, frontline antibodies your body produces when it first encounters a new infection. High IgM levels are a strong indicator of a current or very recent infection. This test is a critical tool for your doctor to determine if your symptoms are from an acute, active illness. A simple blood draw provides this vital, real-time snapshot of your immune system in action.