Immunoglobulin A is an antibody that plays a crucial role in the immune function of mucous membranes. The amount of IgA produced in association with mucosal membranes is greater than all other types of antibody combined. In general terms, between three and five grams are secreted into the intestinal lumen each day. This represents up to 15% of total immunoglobulins produced throughout the body. An IgA test measures the blood level of immunoglobulin A. Antibodies are proteins made by immune system to fight bacteria, viruses and toxins. It is found in high concentrations in respiratory passages and gastrointestinal tract, as well as saliva and tears. It also plays a role in allergic reactions. IgA levels may be high in autoimmune conditions, disorders in which body mistakenly makes antibodies against healthy tissues.