What is Immunoglobulin ‘A’ (IgA)?

Immunoglobulin A type is the most important immunoglobulin fundamentally in mucosa immunity and lactation. They are predominantly seen in seromucosal secretions such as saliva, nasal fluids, sweat, colostrum, and cretin of respiratory, gastrointestinal, urinogenital tract etc., to act at different levels. They help in preventing the attachment of pathogens to the walls, and neutralizing the activity of some viruses, both inside and outside epithelium cells.

Since they are resistant to degradation by enzymes, secretary IgA can survive in harsh environments such as the digestive and respiratory tracts, to provide protection against microbes that multiply in body secretions. They constitute about 10-15% of total immunoglobulins.

The daily production of secretary IgA is greater than that of any other immunoglobulin class. Every day human mucosa produces 5 to 15gms of secretary IgAs. IgAs are poor activators of the complement system, and opsonises faintly.

In general IgA molecules are monomers in their basic structure with 8-10% of carbohydrate content.

Each IgA monomer molecule encompass two identical heavy chains of alpha type and two identical light chains of kappa or lambda just as other immunoglobulins do.

The molecular weight of IgA monomer is 1, 50,000 Daltons or 150 kD, sedimentation coefficient is 6.85, and half life is 6-8 days. Since IgA molecules express morphological diversity in different locations, the secretary IgA is much larger than serum IgA.

Formation of dimmers during secretion through a J chain makes the secretary IgA molecules larger than the plasma IgAs (J chain is a cysteine rich polypeptide with a molecular mass of 15kD).

Prior to secretion, IgA produced by plasma cells in the lamina propria (adjacent to mucosal surfaces) bound to poly Ig receptors present on the basolateral membranes of mucosal epithelial cells. IgAs bound to this receptor are internalized through receptor mediated endocytosis, the poly Ig receptors cleave from membrane and becomes secretary component to release in to the lumen as IgA polymer.

The secretary component masks protease susceptible regions of hinge region to prevent cleavage of IgA molecules in protease rich mucosal environment. Since IgA molecules are helping in the protection of mucosal membrane, IgA immunoglobulins are referred as "antiseptic paint of mucosal membrane".

IgA immunoglobulins secreted along with breast milk provide protection to the gastro intestinal tract of new born babies. Hence the health secret of breast fed babies is presence of IgA immunoglobulins in breast milk.

In blood circulation IgA interacts with an Fc receptor called Fca RI (or CD89) expressed on immune effector cells, to initiate inflammatory reactions. Ligation of Fc a RI by IgA containing immune complexes induces various immunological responses such as antibody-dependent cell-mediated cytotoxicity (ADCC), degranulation of eosinophils and basophils,

Two sub classes of IgA molecules namely IgAl and IgA2 exist in mammals. IgA2 is completely devoid of interring chain disulphide bonds and the four polypeptide chains are held together by non covalent linkages.

IgAl is found in serum and made by bone marrow B cells. IgA2 is made by B cells located in the mucosae and has been found to secrete into colostrum, breast milk, tears and saliva.