Immunoglobulin G (IgG) is the most abundant antibody in the blood and plays a crucial role in the body's immune defense system. This test measures IgG levels to evaluate humoral immunity, which is the body's ability to produce antibodies against infections. IgG antibodies provide long-term protection against bacteria, viruses, and other pathogens. Low IgG levels can indicate immunodeficiency disorders, making patients more susceptible to recurrent infections. High IgG levels may suggest active infection, inflammation, or certain blood disorders like multiple myeloma. This test is commonly ordered when evaluating patients with frequent infections, suspected immune deficiencies, or when monitoring treatment for conditions like IgG myeloma. IgG levels are also important for assessing the effectiveness of immunoglobulin replacement therapy in patients with antibody deficiencies.