Calprotectin: the pathway in the body · Biomarker & Stool Analysis

This page shows the biochemical pathway behind the laboratory value Calprotectin (stool): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Calprotectin is a protein made of two subunits, S100A8 and S100A9. It is abundant in neutrophil granulocytes and binds metal ions such as zinc and manganese outside the cell. In this way it withholds metals from microbes and signals damaged tissue to immune 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. Neutrophils → Calprotectin in cytosol · Calcium Inside these cells, calprotectin is present in large amounts. It consists of the proteins S100A8 and S100A9, which pair up; calcium stabilises the pair. It forms a store that later acts outside. Source 1, 2↑ supplies In the cytosol, calprotectin makes up a large share of the soluble proteins of neutrophils. It binds calcium there and forms a store that reaches the outside when the cell is activated. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If a lot of S100A8 and S100A9 is made, for example in response to inflammatory messengers, more calprotectin lies in the cell, and correspondingly more of it is available outside on release.

   too little — If little S100A8 and S100A9 is made, little calprotectin reaches the outside on release; in animal models without S100A9 the pair is missing entirely.

   observed in studies · Source 1

2. Calprotectin in cytosol → Migration into tissue · Signalling molecules Signalling molecules from the tissue make neutrophils stick to the vessel wall. They squeeze between the cells and migrate to the site of inflammation, for example into the gut wall. The more messengers, the more cells cross over. Source 4↑ supplies Migration brings neutrophils, and calprotectin with them, to where messengers signal irritation. The more messengers the tissue releases, the more cells leave the vessel. established physiology Source 4
   ⚖ When the balance tips

   too much — If the messengers keep attracting them, ever more neutrophils cross into the tissue and accumulate there; with them, the amount of calprotectin on site rises.

   too little — If neutrophils barely stick to the vessel wall, for example because adhesion molecules are missing, they stay in the blood and do not reach the tissue.

   established physiology · Source 4

3. Migration into tissue → Release On strong activation and when the cell breaks down, calprotectin reaches the outside. The DNA nets that neutrophils cast out also carry it with them. Only outside can it bind metals. Source 1, 4↑ supplies Only release turns the store inside the cell into a substance that acts outside: calprotectin reaches tissue, secretions and gut contents when neutrophils are strongly activated or break down. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — If many neutrophils break down or cast out many DNA nets, plenty of calprotectin reaches the outside and is available there for metal binding and signalling.

   too little — If neutrophils are barely activated, calprotectin stays largely inside the cell, and little of it arrives outside.

   established physiology · Source 1, 4

4. Release → In the gut contents Once calprotectin reaches the gut, it persists there for some time because digestive enzymes break it down only slowly. This is why it can be measured in stool. There, too, it binds zinc and manganese. Source 1↓ depletes In the gut contents, too, calprotectin binds zinc and manganese and withholds them from the microbes living there. Because it is broken down only slowly, the amount in stool depends on how many neutrophils have reached the gut wall. observed in studies Source 1, 2
   ⚖ When the balance tips

   too much — If a lot of calprotectin accumulates in the gut contents, more zinc and manganese are bound there; microbes with strong uptake systems of their own for metals cope better with this than others.

   too little — If hardly any neutrophils reach the gut wall, little calprotectin is found in the gut contents, and zinc and manganese remain more available to microbes there.

   observed in studies · Source 1, 2

5. Calprotectin outside → Zinc and manganese held · Calcium Between the two subunits lie two binding sites. They grip zinc and manganese very tightly; iron and nickel are bound there too. Calcium makes the binding tighter. Hardly any remains free. Source 2↓ depletes The metal pockets hold zinc and manganese so tightly that hardly any remains free around them. Calcium, which is plentiful outside the cell, makes this binding even tighter. established physiology Source 2
   ⚖ When the balance tips

   too much — If a lot of calprotectin is present, more zinc, manganese and iron are bound, and the free amount of these metals in the surroundings falls.

   too little — If little calcium is bound to calprotectin, the pockets hold the metals less tightly; inside the cell, where little calcium is free, it therefore binds less tightly than outside.

   observed in studies · Source 2

6. Zinc and manganese held → Metals withheld Bacteria and fungi need zinc and manganese for their own enzymes. When the ions are bound, less of them is available to these microbes. This principle is called nutritional immunity. Their metal-dependent enzymes then work more slowly. Source 2↓ depletes If little zinc and manganese is free, enzymes of bacteria and fungi slow down that they need for growth and for breaking down oxygen radicals, such as their manganese superoxide dismutase. observed in studies Source 2
   ⚖ When the balance tips

   too much — If the metals are strongly withheld, many bacteria switch on their own particularly strong uptake systems; species with such systems then cope better than others.

   too little — If the metals are barely bound, zinc and manganese remain available to microbes, and their metal-dependent enzymes work unhindered.

   observed in studies · Source 2

7. Calprotectin outside → Signal to immune cells Calprotectin attaches to the receptors TLR4 and RAGE. The cell reads this as a sign of damaged tissue and then forms further signalling molecules. These attract further neutrophils. Source 3, 1↕ both, depending on amount Via TLR4, calprotectin makes immune cells produce messengers such as TNF. Briefly, this summons further defence cells; if the signal persists, it feeds inflammation further. Parts of this come from animal models. observed in studies Source 3, 1
   ⚖ When the balance tips

   too much — If a lot of calprotectin stays bound to TLR4, the cells keep producing messengers that attract new neutrophils; these bring more calprotectin with them, and the response feeds itself.

   too little — If little calprotectin reaches the receptors, immune cells produce fewer messengers; in animal models without calprotectin, the response to endotoxin is weaker.

   observed in studies · Source 1, 3

Further stations

• Neutrophils — a type of white blood cell Neutrophil granulocytes are the most common white blood cells. They are formed in the bone marrow and circulate in the blood. At a site of inflammation they break down microbes. Source 4↑ supplies Neutrophils are the first defence cells at a site of inflammation: they engulf microbes and break them down with enzymes and reactive oxygen species. They bring their store of calprotectin with them. established physiology Source 4
  ⚖ When the balance tips

  too much — If many neutrophils migrate in and stay for long, they also release enzymes and reactive oxygen species into the surrounding tissue, which is altered in the process.

  too little — If there are few neutrophils in the blood, fewer defence cells reach the site of irritation, and less calprotectin arrives there with them.

  established physiology · Source 4

• Calprotectin outside — in tissue and secretions Outside the cell, calprotectin does two things: it binds metal ions, and it attaches to receptors on immune cells. If the signal lasts long, inflammation continues. Source 1↕ both, depending on amount Outside the cell, calprotectin acts in two directions: it withholds metals from microbes, and it signals damaged tissue to immune cells. In small amounts both are part of defence; in large amounts it keeps inflammation going. observed in studies Source 1, 3
  ⚖ When the balance tips

  too much — If calprotectin builds up in the tissue, signalling via TLR4 and RAGE keeps the production of messengers going; in animal models the inflammatory response is then stronger.

  too little — If there is little calprotectin outside, more zinc and manganese remain free, and the signal to immune cells is weaker; in animal models lacking the protein, the response to bacterial components is weaker.

  observed in studies · Source 1, 3

Cofactors in this pathway

• Calcium — Holds the two subunits together and tightens the binding of zinc and manganese, especially outside the cell Source 1, 2In the ORY catalogue as a laboratory value: Calcium (intrazellulär)
• Zinc — Bound very tightly at the two metal pockets between the subunits and thus withheld from microbes Source 2In the ORY catalogue as a laboratory value: Zink
• Manganese — Occupies the second metal pocket of calprotectin together with zinc; microbes then lack it for their enzymes Source 2
• Iron — Bound at the same metal pocket as manganese and thus withheld from microbes Source 2In the ORY catalogue as a laboratory value: Eisen
• Nickel — Also binds at the pocket between S100A8 and S100A9 and is thus withheld from microbes Source 2

Sources

1. Jukic A, Bakiri L, Wagner EF et al. Calprotectin: from biomarker to biological function. Gut 2021 · PubMed 34145045
2. Zygiel EM, Nolan EM. Transition Metal Sequestration by the Host-Defense Protein Calprotectin. Annu Rev Biochem 2018 · PubMed 29925260
3. Vogl T, Tenbrock K, Ludwig S et al. Mrp8 and Mrp14 are endogenous activators of Toll-like receptor 4, promoting lethal, endotoxin-induced shock. Nat Med 2007 · PubMed 17767165
4. Kolaczkowska E, Kubes P. Neutrophil recruitment and function in health and inflammation. Nat Rev Immunol 2013 · PubMed 23435331

Related pathways

• ASCA and ANCA — neutrophils
• Clostridioides difficile — neutrophils
• Candida — neutrophils
• Granulocytes in the gut wall — neutrophils
• Histamine — release

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