Immunoglobulin A
Overview
Immunoglobulin A (IgA) is a type of 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 absolute 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.
Structure
IgA has two subclasses (IgA1 and IgA2) and can be produced as a monomeric as well as a dimeric form. The IgA dimeric form is the most prevalent and is also called secretory IgA (sIgA). The secretory component of sIgA protects the immunoglobulin from being degraded by proteolytic enzymes, thus sIgA can survive in the harsh gastrointestinal tract environment and provide protection against microbes that multiply in body secretions.
Function
IgA has two separate functions in the immune system. First, it is the first line of defense against invading microbes at the mucosal surfaces of the body. Second, IgA is also involved in the development of immune tolerance to antigens in the gut. It does this by binding to antigens and neutralizing them without the production of inflammation. This is a key feature of IgA, as it allows the immune system to respond to antigens without causing tissue damage.
Role in Disease
Alterations in the serum IgA levels have been observed in various diseases. For example, IgA deficiency is the most common immunodeficiency disorder in humans, affecting approximately 1 in 700 individuals. This condition can lead to recurrent infections, particularly of the mucous membranes. On the other hand, elevated levels of IgA can be indicative of chronic inflammatory diseases, such as Inflammatory Bowel Disease (IBD) and Celiac Disease.
Clinical Significance
In clinical practice, measurement of IgA levels in the blood can be used as a diagnostic tool. For example, low levels of IgA can indicate an IgA deficiency, while high levels can indicate a state of chronic inflammation or infection. Furthermore, specific IgA antibodies can be measured to diagnose certain conditions. For example, the presence of IgA antibodies against the tissue transglutaminase enzyme is a key diagnostic marker for celiac disease.