Immunoglobulin A (IgA) is an antibody that is a component of the immune system. It is the most abundantly produced antibody in the body, with three to five grams secreted into the intestinal lining alone each day. This significant output highlights its specialized function in protecting the body’s extensive mucosal surfaces. These surfaces, which line the respiratory, gastrointestinal, and genitourinary tracts, are the primary entry points for many pathogens, where IgA acts as a first line of defense.
The Structure and Location of Immunoglobulin A
Immunoglobulin A exists in two primary structural forms. In the bloodstream, IgA circulates predominantly as a monomer, a single Y-shaped unit composed of two heavy chains and two light chains. This serum IgA is involved in systemic immune responses, while the IgA found at mucosal surfaces is a more complex structure known as secretory IgA (sIgA).
The formation of secretory IgA begins inside plasma cells located near mucosal tissues. Here, two IgA monomers are linked by a joining (J) chain to form a dimer. Once secreted from the plasma cell, this dimeric IgA binds to a receptor on nearby epithelial cells. This triggers the transport of the IgA dimer across the epithelial cell and its release onto the mucosal surface.
During this transport, a piece of the receptor, called the secretory component, remains attached to the IgA dimer. This secretory component acts as a protective shield, making the sIgA molecule resistant to degradation by the enzymes and harsh conditions found in environments like the digestive tract. This structural feature allows sIgA to function effectively in secretions such as saliva, tears, sweat, and the mucus lining the gut and airways.
Primary Functions in Mucosal Immunity
At mucosal surfaces, the principal role of secretory IgA (sIgA) is to perform immune exclusion. This is a process where the antibody acts as a barrier, preventing pathogens like bacteria and viruses from attaching to and penetrating epithelial cell linings. By binding to antigens on the surface of these invaders, sIgA neutralizes them before they can cause infection, an action akin to blocking intruders at a gateway.
The structure of sIgA, with its multiple binding sites, allows it to agglutinate pathogens, clumping them together. This cross-linking process traps the aggregated microbes within the mucus layer. Once ensnared in the mucus, the pathogens can be cleared from the body through natural processes like peristalsis in the gut or the ciliary escalator in the respiratory tract. This mechanism physically removes threats without initiating a strong inflammatory reaction.
This non-inflammatory method of pathogen clearance is a defining feature of IgA-mediated immunity. While other immune responses can cause inflammation and tissue damage, sIgA is designed to handle constant exposure to foreign substances in a controlled manner. It protects mucosal areas without triggering harmful inflammatory responses that could disrupt tissue function. This allows the immune system to maintain a peaceful coexistence with commensal bacteria in the gut while selectively targeting harmful organisms.
The Role of IgA in Infant Health
The transfer of Immunoglobulin A from mother to child through breast milk is an important aspect of newborn health. IgA is the main antibody found in human milk, with particularly high concentrations in colostrum, the nutrient-rich fluid produced in the first few days after birth. This process provides the infant with passive immunity, as the mother passes on her own matured antibodies for immediate protection.
This maternal IgA guards the infant’s developing gastrointestinal and respiratory systems. The secretory IgA (sIgA) in breast milk is not absorbed into the infant’s bloodstream; instead, it coats the mucosal surfaces of the baby’s gut. There, it performs its function of immune exclusion, neutralizing pathogens the infant may ingest.
This protection is important during the first months of life when the infant’s own immune system is not yet fully functional. The sIgA from the mother’s milk helps to prevent common illnesses and shapes the development of the infant’s gut microbiome. It selectively allows beneficial bacteria to flourish while preventing harmful ones from causing disease, supporting the child’s long-term immune function.
Clinical Significance of IgA
The most common primary immunodeficiency is Selective IgA Deficiency, a condition characterized by low or absent levels of IgA in the blood and at mucosal surfaces. Many individuals with this deficiency are asymptomatic and may never be diagnosed. However, others can experience an increased frequency of infections, particularly in the respiratory, gastrointestinal, and genitourinary tracts.
Without sufficient IgA to perform immune exclusion at mucosal sites, these individuals may be more susceptible to recurrent illnesses. The absence of this antibody means pathogens have an easier time adhering to and invading surface tissues. While other components of the immune system often compensate, the protective barrier is compromised, leading to a pattern of recurring health issues for some patients.
In some cases, IgA itself can be directly involved in disease processes. One example is IgA nephropathy, also known as Berger’s disease. In this autoimmune condition, abnormal, poorly glycosylated IgA1 molecules accumulate in the kidneys. These IgA deposits form immune complexes that trigger inflammation and damage to the glomeruli, the tiny filtering units within the kidneys, which can impair kidney function and potentially lead to chronic kidney disease.