Sekretorisches IgA (sIgA): the pathway in the body · Biomarker & Stool Analysis

Sekretorisches IgA (sIgA) is part of the pathway “Secretory IgA”. This page shows the whole pathway; the station of Sekretorisches IgA (sIgA) is highlighted.

Where this laboratory value sits: Secretory IgA — with secretory component. At the gut surface, the receptor is cleaved. A piece of it stays attached to the IgA: the secretory component. It makes the molecule resistant to digestive enzymes. It thus stays functional in the gut for a long time. Source 2, 3

In brief

Secretory IgA is an antibody made of two IgA molecules, a J chain and the secretory component. It lies in the mucus layer of the gut and binds bacteria, viruses and food components there. There it keeps them away from the gut cells.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Peyer's patches → B cell with IgA Class switching · TGF-β, APRIL, BAFF Signalling molecules such as TGF-β, APRIL and BAFF direct class switching: the B cell changes the type of its antibody from IgM to IgA. This choice also holds for its descendants. Source 1↑ supplies Class switching fixes that the cell and its descendants make IgA instead of IgM. The enzyme AID rearranges the antibody's genetic information for this; TGF-β, APRIL and BAFF give the signal. established physiology Source 1
   ⚖ When the balance tips

   too much — If TGF-β, APRIL and BAFF are plentiful, more B cells switch to IgA; retinoic acid from vitamin A drives this switch further.

   too little — If little retinoic acid is available, fewer B cells switch to IgA in the animal model, and fewer of them carry the homing receptors that guide them into the gut mucosa.

   observed in studies · Source 1, 4

2. B cell with IgA → Plasma cell The B cell matures into a plasma cell and migrates into the layer beneath the gut epithelium. There it continuously releases IgA. The gut mucosa holds more such cells than any other tissue. Source 1↑ supplies The plasma cell turns its metabolism over entirely to making IgA and releases it continuously. The gut mucosa holds more plasma cells than any other tissue of the body. established physiology Source 1
   ⚖ When the balance tips

   too much — If gut bacteria supply many short-chain fatty acids, B cells in the animal model have more acetyl-CoA and energy available; more of them become plasma cells, and they release more antibodies.

   too little — If few short-chain fatty acids arrive, B cells in the animal model form fewer plasma cells, and less IgA is made in the gut.

   observed in studies · Source 7

3. Plasma cell → Dimeric IgA · J chain The plasma cell links two IgA molecules via a small protein, the J chain. Only this double form fits the transport receptor of the gut cell. Without the J chain, IgA stays in the tissue. Source 2↑ supplies Only the J chain makes IgA transportable: it is the binding site for the pIgR of the gut cell. Single IgA without a J chain stays in the tissue or passes into the blood. established physiology Source 2
   ⚖ When the balance tips

   too much — If the mucosa forms more dimeric IgA than there are pIgR molecules, the rest stays in the tissue; the number of receptors limits transport.

   too little — If little dimeric IgA is available, the gut cell still ferries the pIgR across; it then reaches the mucus without cargo, as free secretory component.

   established physiology · Source 2

4. IgA at the pIgR → Route through the cell The cell ferries the complex in small vesicles from one side to the other. This route through the cell is called transcytosis. The receptor does not return. Source 2↑ supplies Transcytosis carries IgA from the blood side to the gut side without opening a gap between the cells. The route runs one way only: the receptor does not return. established physiology Source 2
   ⚖ When the balance tips

   too much — If many IgA complexes pass through the cell, in cell culture they can bind viruses that have entered the cell on the way and carry them out.

   too little — If little IgA is ferried through, correspondingly less reaches the gut surface; the route does not get faster, because it depends on the number of receptors.

   observed in studies · Source 2, 3

5. Route through the cell → Secretory IgA Proteases At the gut surface, the receptor is cleaved. A piece of it stays attached to the IgA: the secretory component. It makes the molecule resistant to digestive enzymes. It thus stays functional in the gut for a long time. Source 2, 3↑ supplies The secretory component covers the sites where digestive enzymes would attack. sIgA thus stays functional for a long time in the protein-splitting environment of the gut, as experiments with purified enzymes show. observed in studies Source 3, 2
   ⚖ When the balance tips

   too much — If much sIgA forms, it collects in the mucus; whatever is not held there moves on with the gut contents.

   too little — If IgA carries no secretory component, digestive enzymes break it down faster, and it stays in the mucus for a shorter time.

   observed in studies · Source 3

6. Secretory IgA → sIgA in the mucus The finished sIgA lies in the mucus layer covering the gut cells. The sugar chains of the secretory component hold it there. There it binds without triggering inflammatory messengers. Source 3↑ supplies In the mucus, sIgA intercepts microbes and food components before they reach the gut cells. It binds without triggering inflammatory messengers, because IgA barely activates the complement system. established physiology Source 3
   ⚖ When the balance tips

   too much — If much sIgA lies in the mucus, more bacteria are coated with IgA; in the animal model this coating influences which species stay close to the mucosa.

   too little — If little sIgA lies in the mucus, more bacteria reach the gut cells in the animal model, and the defences in the mucosa are called on more.

   observed in studies · Source 3, 1

7. sIgA in the mucus → Binding of microbes sIgA attaches to bacteria, viruses and food components. Clumped together, they stay in the mucus and reach the gut cells less often; this process is called immune exclusion. The mucosa thus receives a trigger for inflammatory defence less often. Source 3↕ both, depending on amount sIgA occupies binding sites on microbes and toxins, so that they dock onto the gut cells less often. At the same time, binding helps some resident bacteria to stay in the mucus. observed in studies Source 3
   ⚖ When the balance tips

   too much — If many microbes are loaded with sIgA, they clump together and are carried away with the gut contents instead of reaching the gut cells.

   too little — If little sIgA is available for binding, more microbes and food components reach the gut cells; parts of the immune system that work with inflammatory messengers then step in.

   observed in studies · Source 3

8. Binding of microbes → sIgA in the stool Part of the sIgA leaves the gut with the stool, where it can be measured. Bound microbes leave the gut with it. Source 3↓ depletes sIgA and the microbes bound to it leave the gut with the stool. In this way the body removes clumped bacteria and food components from the gut contents. established physiology Source 3
   ⚖ When the balance tips

   too much — If more sIgA is formed or bound more strongly to microbes, more of it reaches the stool; the amount there reflects formation and passage together.

   too little — If little sIgA is formed or ferried through the gut cell, little arrives in the stool as well; the amount alone does not show which step this comes from.

   established physiology · Source 3

Further stations

• Peyer's patches — lymphoid tissue in gut wall Clusters of immune cells lie in the gut wall. There, B cells meet components of bacteria and food that are shuttled in from the gut lumen. This is where the IgA response begins. Source 1↑ supplies This is where the IgA response begins: specialised cells of the gut wall shuttle in components from the gut lumen, and dendritic cells present them to B cells. Germinal centres form, in which B cells mature. established physiology Source 1
  ⚖ When the balance tips

  too much — If many components of gut bacteria arrive, more germinal centres form in the Peyer's patches, and more B cells are switched to IgA.

  too little — If few bacterial components arrive, as in the germ-free animal model, the Peyer's patches stay small and few IgA-producing plasma cells form.

  observed in studies · Source 1

• IgA at the pIgR — receptor of the gut cell The polymeric immunoglobulin receptor (pIgR) sits on the blood-facing side of the gut cell. The IgA pair binds to it. Each receptor carries only one cargo and is used up doing so. Source 2↑ supplies The pIgR is the means of transport for IgA through the gut cell. Each receptor molecule carries exactly one cargo and is used up on the way; how much IgA gets out therefore depends on the amount of pIgR. established physiology Source 2
  ⚖ When the balance tips

  too much — Signals from gut bacteria and immune messengers such as TNF-α and interferon-γ make gut cells in cell culture form more pIgR; more IgA is then ferried through the cell.

  too little — If the gut cell forms little pIgR, as in the animal model without the pIgR gene, dimeric IgA builds up in the tissue and blood, and hardly any sIgA reaches the mucus.

  observed in studies · Source 2

Cofactors in this pathway

• Vitamin A — Retinoic acid is formed from it and steers the class switch of the B cell to IgA and guides the cell into the gut mucosa Source 4
• Acetate — Binds the GPR43 receptor of the B cell, a route involved in the class switch to IgA; in the animal model IgA rises Source 6
• Acetyl-CoA — Formed in the B cell from short-chain fatty acids and supplies building blocks for antibodies and energy Source 7
• Vitamin D — The active form makes maturing B cells express the homing receptor CCR10, which guides them to the mucosa Source 5In the ORY catalogue as a laboratory value: Vitamin D

Sources

1. Gutzeit C, Magri G, Cerutti A. Intestinal IgA production and its role in host-microbe interaction. Immunol Rev 2014 · PubMed 24942683
2. Johansen FE, Kaetzel CS. Regulation of the polymeric immunoglobulin receptor and IgA transport: new advances in environmental factors that stimulate pIgR expression and its role in mucosal immunity. Mucosal Immunol 2011 · PubMed 21956244
3. Mantis NJ, Rol N, Corthésy B. Secretory IgA's complex roles in immunity and mucosal homeostasis in the gut. Mucosal Immunol 2011 · PubMed 21975936
4. Mora JR, von Andrian UH. Role of retinoic acid in the imprinting of gut-homing IgA-secreting cells. Semin Immunol 2009 · PubMed 18804386
5. Shirakawa AK, Nagakubo D, Hieshima K et al. 1,25-dihydroxyvitamin D3 induces CCR10 expression in terminally differentiating human B cells. J Immunol 2008 · PubMed 18292499
6. Wu W, Sun M, Chen F et al. Microbiota metabolite short-chain fatty acid acetate promotes intestinal IgA response to microbiota which is mediated by GPR43. Mucosal Immunol 2017 · PubMed 27966553
7. Kim M, Qie Y, Park J et al. Gut Microbial Metabolites Fuel Host Antibody Responses. Cell Host Microbe 2016 · PubMed 27476413

Whole pathway: Secretory IgA

Related pathways

• ACE2 and GM-CSF autoantibodies — plasma cell
• ASCA and ANCA — plasma cell
• IgG antibodies to food — plasma cell

As of 2026-09-16. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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