Immunoglobulin A, or IgA, is an antibody that plays a significant role in the body’s defense system. As a protein produced by the immune system, IgA helps protect against foreign substances. It is one of five major classes of antibodies found in humans, contributing to the overall immune response.
IgA: The Body’s Mucosal Guardian
IgA is predominantly found in the body’s mucosal secretions, which are moist linings acting as barriers against the external environment. These include saliva, tears, breast milk, and the mucous membranes of the gastrointestinal, respiratory, and genitourinary tracts. The amount of IgA produced in association with mucosal membranes surpasses all other antibody types combined, with an estimated three to five grams secreted into the intestinal lumen daily.
A specialized form, secretory IgA (sIgA), is well-suited for these environments. sIgA is a dimeric molecule, consisting of two IgA units linked by a J-chain, and includes a secretory component (SC). This component protects sIgA from degradation by enzymes in harsh environments, such as the digestive tract. This unique structure enables sIgA to act as a primary defense at vulnerable entry points, preventing harmful substances from entering the body.
How IgA Protects the Body
IgA provides protection through several mechanisms, primarily by preventing pathogens from establishing infections at mucosal surfaces. It can directly bind to viruses, bacteria, and toxins, neutralizing their harmful effects. By coating these pathogens, IgA prevents them from attaching to and invading host cells.
Beyond direct neutralization, IgA can also aggregate pathogens, clumping them together for easier removal. This process, known as immune exclusion, traps pathogens in mucus, limiting their ability to reach and penetrate epithelial cells. This mechanism is important in the gastrointestinal tract, where IgA helps maintain a balanced microbiome by binding to dietary antigens and harmless bacteria, preventing an inflammatory response.
IgA contributes to maintaining gut homeostasis by influencing the composition and behavior of the gut microbiota. It can prevent the colonization of pathogenic bacteria and promote the growth of beneficial commensal microbes. This multifaceted action ensures that harmful invaders are dealt with, while the body’s own beneficial residents are not unnecessarily targeted, supporting overall health without triggering excessive inflammation.
When IgA Levels Are Imbalanced
Imbalances in IgA levels, either too low or too high, can indicate underlying health conditions. Selective IgA deficiency (SIgAD) is the most common primary immunodeficiency, characterized by absent or very low IgA levels while other immunoglobulins remain normal. Many individuals with IgA deficiency experience no symptoms, but some are more susceptible to recurrent infections, particularly affecting the eyes, ears, sinuses, respiratory, and gastrointestinal systems.
Diagnosis of IgA deficiency involves blood tests showing undetectable IgA levels, usually less than 7 mg/dL, alongside normal levels of other antibodies like IgG and IgM. This condition can also be associated with an increased risk of autoimmune diseases, such as rheumatoid arthritis, lupus, and celiac disease, as well as allergies and asthma. While there is no specific cure, management often focuses on treating infections with antibiotics and addressing associated conditions.
Conversely, elevated IgA levels can signal various health concerns. These include chronic infections, certain autoimmune disorders like IgA nephropathy, and liver diseases such as alcoholic cirrhosis. Some cancers, such as multiple myeloma or lymphoma, may also be associated with very high IgA levels due to abnormal immune cell production. An elevated IgA level is not a diagnosis itself, but an indicator that prompts further medical evaluation to identify and address the underlying condition.