Immunoglobulin A (IgA) is the second most abundant class of immunoglobulin next to IgG, constituting about 10-15% of total serum immunoglobulin and it is the predominant immunoglobulin class in external secretions.
IgA can exist as a monomer, dimer, trimer, or tetramer. IgA in serum (also called serum IgA) is predominantly in monomeric form. Secretory IgA, which is a dimeric form of IgA, is the predominant antibody found in body secretions such as breast milk, saliva, tears, and mucus of the intestinal and respiratory and genitourinary tract.
Structure of Immunoglobulin A (IgA)
Immunoglobulin A (IgA) consists of two α heavy chains and two κ or two λ light chains with molecular formula (α2κ2)n or (α2λ2)n, where n =1, 2, 3 or 4. In humans, there are two subclasses of α chains-α1 and α2 and thus two subclasses, IgA1 and IgA2. IgA1 is present mainly in the serum (about 85% of serum IgA) and secretions of mucosal glands & upper intestine whereas IgA2 predominates in the secretions of the large intestine and female genital tract.
IgA monomer contains three constant-region domains (CH1, CH2, and CH3) and a hinge region. Secretory IgA consists of at least two IgA molecules, which are covalently linked to each other through a J chain. In addition, there is another joining segment present between two IgA molecules called secretory component. Secretory component helps the dimeric IgA to cross the epithelial surface (transcytosis) to reach the lumen. It also protects IgA from denaturation by bacterial proteases.
Functions of Immunoglobulin A (IgA)
IgA has a variety of functions. The main functions of IgA is carried by SIgA while the function of circulating IgA is less clear.
Immunoglobulin A (IgA) is the most prevalent antibody class in exocrine secretions where it provides the first line of immune defense against foreign microorganisms. sIgA present in mucosal surfaces can bind with bacterial and viral surface antigens forming sIgA-Ag complexes which are eliminated by the ciliated epithelial cells of the respiratory tract or by peristalsis of the gut. SIgA thus prevents attachment of the pathogens to the mucosal cells, inhibiting subsequent colonization and infection.
Some pathogenic bacteria of the respiratory tract such as Streptococcus pneumoniae, Haemophilus influenzae secrete protease enzymes (IgA protease) that cleave IgA dimers present in the mucosal secretions, thus reducing the effectiveness of IgA.
Immunoglobulin A (IgA)
does not activate the classical complement pathway
does not cross the placenta
Is not present on the membrane of mature B cells
does not bind to Fc receptors of phagocyte
does not induce mast-cell degranulation
Passive immunity to newborn babies
Breast milk (especially colostrums) is rich in secretory IgA and many other molecules that help to protect the newborn against infections during the first few months of life. Because the immune system of infants is not fully functional, breastfeeding plays an important role in maintaining the health of newborns by protecting the immunologically immature infant gut.
References and further readings
- Cellular and Molecular Immunology, 9th Edition
- Kuby Immunology, 8th Edition
- Roitt’s Essential Immunology, 13th Edition