Written and maintained by CASRAI Editorial Board
Last updated
Short answer
Secretory IgA (SIgA) is the form of immunoglobulin A found at mucosal surfaces such as the gut lining, airways, saliva and milk. It is mostly a dimer: two IgA molecules joined by a J chain and wrapped in a fragment of the polymeric immunoglobulin receptor called secretory component. It works mainly by physically coating and clumping microbes, not by triggering inflammation. This page is general scientific information, not medical advice, and it does not interpret anyone’s test result.
Definition and scope
IgA is one of the antibody classes (see antibody isotyping for how classes are told apart). Serum IgA and mucosal IgA are not the same molecular form. Sousa-Pereira and Woof (2019) describe the main form at mucosal surfaces, secretory IgA, as dimeric, with some tetramers; the eLife structural paper (2020) calls it the predominant mucosal antibody, binding both pathogens and commensal microbes.
SIgA is therefore a complex of three kinds of component, not a single protein:
| Component | What it is | Source |
|---|---|---|
| IgA (two copies in a dimer) | The antibody units; a dimer gives four antigen-binding Fab arms | eLife 2020 |
| J chain (joining chain) | A 15 kDa polypeptide that links the IgA units into a polymer | Sousa-Pereira and Woof 2019; eLife 2020 |
| Secretory component (SC) | The ectodomain of the polymeric Ig receptor (pIgR) that stays bound to the antibody after transport | eLife 2020 |
How it is made and how it works
Per the eLife structural study, dimeric IgA is bound by pIgR on the basolateral side of epithelial cells and carried across the cell (transcytosis). At the apical surface, pIgR is cleaved and its ectodomain is released still attached to the antibody, which is how SIgA is formed. pIgR binds only polymeric immunoglobulins, so monomeric IgA is not transported this way (Sousa-Pereira and Woof 2019). The secretory component is thought to help protect the antibody from degradation in the mucosal environment.
The same sources describe the action as largely physical: coating, cross-linking, agglutination and “enchained growth” of microbes, typically without inflammation. Because a dimer carries multiple antigen-binding sites, it can bind with high avidity and cross-link targets; agglutination keeps pathogens from penetrating mucus, a process called immune exclusion. The structure of the Fc domain is what distinguishes the IgA heavy chain from other classes.
Types: IgA1, IgA2, dimers and tetramers
| Feature | What the source states | Source |
|---|---|---|
| Subclass mix at mucosal sites | About 40% IgA1 and 60% IgA2 | Sousa-Pereira and Woof 2019 |
| Subclass mix in serum | About 90% IgA1 and 10% IgA2 | Sousa-Pereira and Woof 2019 |
| Hinge and bacterial proteases | IgA1 has a longer hinge that bacterial IgA1 proteases can cleave; IgA2 generally is not cleaved | Sousa-Pereira and Woof 2019 |
| Polymer size in saliva and milk | Dimer to tetramer about 3:2, with 5-10% monomeric | Sousa-Pereira and Woof 2019 |
| Species | Humans have IgA1 and IgA2 (allotypes IgA2m1 and IgA2m2); mice express a single IgA heavy chain | eLife 2020 |
Where it matters for labs: measuring SIgA in saliva
Saliva is the commonest sample for SIgA in research. The practical points below are manufacturer guidance from Salimetrics (a vendor of salivary assays), not a regulatory standard, and your assay’s own instructions take precedence.
- Report a rate, not only a concentration. Salimetrics advises multiplying the assay result (µg/mL) by the flow rate (mL/min) to obtain the secretion rate (µg/min). That requires recording the collection time so flow rate can be calculated.
- Collection method. The vendor states that unstimulated whole saliva collected by passive drool is the best type to use.
- Avoid stimulation. Chewing gum or sour flavours are discouraged because of the dilution effect on SIgA.
- Avoid cotton-based collection materials. The vendor advises that cotton negatively influences SIgA measurements.
For tube and kit contents see the saliva collection kit checklist. Assay formats for antibodies are explained in sandwich ELISA. For the wider field, see what is immunology.
Common misunderstandings
- “IgA in blood is the same as SIgA.” Serum IgA is mostly IgA1 (about 90%), while mucosal IgA is about 40% IgA1 and 60% IgA2, and SIgA is predominantly polymeric with secretory component attached.
- “The 60 mg/kg/day figure is SIgA output.” The source figure of around 60 mg of IgA per kilogram of body weight per day refers to total IgA production, not SIgA specifically.
- “Mouse models mirror human IgA.” Mice express a single IgA heavy chain, whereas humans have IgA1 and IgA2, so subclass-specific findings do not transfer directly.
- “A concentration alone is enough.” Because saliva is diluted or concentrated by flow, vendor guidance is to convert to a secretion rate using flow rate.
- “A level here says something about my health.” This page does not interpret measured levels; that is a question for a clinician.
Sources
- eLife (2020), “The structures of secretory and dimeric immunoglobulin A”: https://elifesciences.org/articles/56098 (read 2026-11-08).
- Sousa-Pereira P, Woof JM. “IgA: Structure, Function, and Developability.” Antibodies (Basel) 2019;8(4):57: https://pmc.ncbi.nlm.nih.gov/articles/PMC6963396 (read 2026-11-08).
- Salimetrics, “Measuring SIgA in saliva” (manufacturer guidance): https://salimetrics.com/measuring-siga-in-saliva/ (read 2026-11-08).
