Antibody Isotypes: IgA

Immunoglobulin A is the dominant antibody for mucosal homeostasis in the gastrointestinal, respiratory, and genitourinary tracts. This means they can be found in body secretions such as tears, saliva, respiratory and intestinal secretions, and makes up 50% of the protein in colostrum. IgA gets produced by class switching of Ig and protects against pathogens by neutralization or prevention from binding to the mucosal surfaces.

Researchers are still debating the exact mechanisms and functions of IgA, as there may be large differences between IgA in different mucosal tissues. For instance, a paper by Pabst & Slack (2020) details the disagreement about the relevancy of T cell-dependent versus T cell-independent IgA in both microbiota and infection control.^1,2

When in serum, IgA is usually a monomer, but in mucosa, secretory IgA takes a dimeric form with a J-chain and a secretory component. Secretory IgA is produced through the functions of plasma cells producing multimeric IgA and epithelial cells producing pIgR. Which form IgA takes does have interesting consequences, for instance, with how polymeric IgA is more effective than monomeric IgA in preventing Clostridium difficile toxins.

IgA can also be further classified into IgA1 and IgA2. IgA1 has a longer hinge region than IgA2, with a duplicated section of amino acids, making it more susceptible to cleavage from bacterial proteases. This may explain why IgA2 is dominant in many mucosal secretions and the colon, while approximately 90% of total IgA in serum is comprised of IgA1.³

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References: 

1. Schroeder, H. W., & Cavacini, L. (2010). Structure and Function of Immunoglobulins. The Journal of Allergy and Clinical Immunology, 125(2 0 2), S41. https://doi.org/10.1016/J.JACI.2009.09.046

2. Pabst, O., & Slack, E. (2019). IgA and the intestinal microbiota: the importance of being specific. Mucosal Immunology 2019 13:1, 13(1), 12–21. https://doi.org/10.1038/s41385-019-0227-4

3. Stubbe, H., Berdoz, J., Kraehenbuhl, J.-P., & Corthésy, B. (2000). Polymeric IgA is superior to monomeric IgA and IgG carrying the same variable domain in preventing Clostridium difficile toxin A damaging of T84 monolayers. Journal of Immunology (Baltimore, Md. : 1950), 164(4), 1952–1960. https://doi.org/10.4049/JIMMUNOL.164.4.1952