What is "immunoglobulin a"

immunoglobulin \im`mu*no*glob"ulin\ ([i^]m`m[-u]*n[-o]*gl[o^]b"[-u]*l[i^]n) n. (1953) any one of a class of globular proteins which are antibodies and are produced by the immune system in animals.

Note: The immunoglobulins form a series of related proteins which are each composed of two pairs of polypeptide chains, called heavy (H) and light (L, meaning of lower molecular weight), all linked together by disulfide bonds. They are subdivided on the basis of the structural and antigenic properties of the H chains into four subgroups, immunoglobulin G (IgG), immunoglobulin A (IgA), immunoglobulin M (IgM), and immunoglobulin D (IgD). They are divided also into subclasses. Both H and L chains of anny given class and subclass have regions which are of constant structure within that class, as well as regions which are of variable structure. The variable regions impart the ability to recognize and bind to specific molecular structures, thus providing the organism the capacity to recognize and defend itself against the harmful effects of substances foreign to the body.
--Stedman

Syn: Ig.

n. one of the most common of the five major classes of immunoglobulins; the chief antibody in the membranes of the gastrointestinal and respiratory tracts [syn: IgA]

Immunoglobulin A (IgA, also referred to as sIgA) is an antibody that plays a critical role in immune function in the mucous membranes. More IgA is produced in mucosal linings than all other types of antibody combined; between three and five grams are secreted into the intestinal lumen each day. This accumulates up to 15% of the total immunoglobulin produced in the entire body.

IgA has two subclasses (IgA1 and IgA2) and can exist in a dimeric form called secretory IgA (sIgA). In its secretory form, IgA is the main immunoglobulin found in mucous secretions, including tears, saliva, sweat, colostrum and secretions from the genitourinary tract, gastrointestinal tract, prostate and respiratory epithelium. It is also found in small amounts in blood. 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. sIgA can also inhibit inflammatory effects of other immunoglobulins. IgA is a poor activator of the complement system, and opsonises only weakly. Its heavy chains are of the type α.