This is the 'master blueprint' of your long-term immune health. The Immunoglobulin G (IgG) test measures the total amount of your body's most common and important 'memory' antibody. IgG is your immune system's primary, long-term defense against bacteria and viruses. A low level can signal an immunodeficiency, while a high level can point to a chronic infection or an autoimmune condition. This simple blood test provides a crucial, top-level view of your immune system's strength and current status, guiding your doctor to the right diagnosis.