Immunoglobulin A (IgA) is an antibody that plays a critical role in mucosal immunity, protecting the body's surfaces that are exposed to the external environment such as the respiratory tract, gastrointestinal tract, and genitourinary system. This test measures IgA levels in the blood to evaluate humoral immunity and diagnose immunodeficiency disorders. IgA deficiency is the most common primary immunodeficiency and can cause recurrent respiratory infections, gastrointestinal problems, and increased susceptibility to allergies and autoimmune diseases. Low IgA levels may also occur in certain blood disorders or as a side effect of certain medications. High IgA levels can indicate active infection, inflammation, or certain blood disorders like IgA myeloma. This test is commonly ordered when evaluating patients with frequent infections, especially of the respiratory or gastrointestinal tracts.