Structure and Forms

• IgA is one of the five main classes of antibodies, existing in two major types: secretory IgA (sIgA) and serum IgA.
• Basic unit: two heavy chains and two light chains, forming a "Y" shape.
• IgA can be dimeric or higher polymers, especially in its secretory form.
• Secretory IgA includes:
• J chain: connects two IgA molecules.
• Secretory component: shields the antibody from hydrolysis in harsh mucosal environments.
• Two types: IgA1 (mainly in blood) and IgA2 (abundant in mucosal secretions).

Production and Distribution

• Most abundantly produced immunoglobulin, exceeding all other antibody classes combined.
• Synthesized by plasma cells in mucosal tissues, particularly gastrointestinal, respiratory, and genitourinary tracts.
• Plasma cells derived from B lymphocytes that have undergone class switching in response to cytokines and T cell help.
• Production of secretory IgA involves:
• Transport of dimeric IgA across epithelial cells via the polymeric immunoglobulin receptor (pIgR).
• Addition of the secretory component, resulting in the complete sIgA molecule.
• Distributed in mucosal secretions, breast milk, and to a lesser extent, in serum.

Functions in Immune Defence

• Acts as the first layer of protection on mucosal surfaces.
• Main function: immune exclusion, preventing adhesion and penetration of pathogens and antigens.
• Functions include:
• Agglutination of microorganisms
• Neutralization of toxins
• Interference with microbial motility
• Involved in the control and regulation of microbial flora, establishing symbiosis with beneficial microorganisms.
• Essential in maternal-infant transmission of immunity, protecting newborns through breast milk.

Clinical Significance and Disorders

• Commonest primary immunodeficiency disorder: IgA deficiency (1 in 500 people of European descent).
• Most individuals with IgA deficiency have no problems, but some may develop repeated respiratory and gastrointestinal infections, allergies, and autoimmune diseases.
• Increased IgA levels linked to:
• IgA nephropathy (most common type of glomerulonephritis)
• Inflammation and potential kidney failure
• Used in celiac disease as a marker (anti-tissue transglutaminase IgA antibodies).
• Focus in vaccine development for pathogens that infect through mucosal routes.