Parasiten (Darm): the pathway in the body · Biomarker & Stool Analysis

Parasiten (Darm) is part of the pathway “Intestinal parasites”. This page shows the whole pathway; the station of Parasiten (Darm) is highlighted.

Where this laboratory value sits: Forms in the stool — DNA detected by PCR. Most of these organisms form a wall again and leave the body with the stool. A PCR test looks for sections of their DNA there and so shows which species is present. With the stool they reach the environment. Source 1, 4, 5

In brief

Intestinal parasites are single-celled organisms such as Giardia lamblia, Entamoeba histolytica or Cryptosporidium. They reach the gut as a dormant form, hatch there and later shed new cysts with the stool. In the gut they attach to the lining or enter its 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. Cysts, oocysts → Excystation · Stomach acid, Bile salts Stomach acid, body warmth and bile salts are the signal to open the wall. The motile forms hatch in the small intestine. So the wall opens only where the organism can live. Source 1, 2↑ supplies The signals ensure that the wall opens only where the organism can live: acid, warmth and bile salts come together only on the way through the stomach and small intestine. observed in studies Source 1, 2
   ⚖ When the balance tips

   too much — If the signals fully coincide, many walls open at once, and many motile forms reach the small intestine.

   too little — If the signals are missing, the wall stays closed, as hatching experiments show, and the dormant form passes through the gut unchanged.

   observed in studies · Source 1, 2

2. Excystation → Active form Giardia lamblia, Entamoeba histolytica, Blastocystis and Dientamoeba fragilis live in the gut as trophozoites. In Cryptosporidium and Cyclospora, the hatching form is called a sporozoite. It lives off the gut contents. Source 1, 2, 5, 6↓ depletes The active form lives off the gut contents: Giardia absorbs nutrients such as cholesterol, which it cannot make itself; Entamoeba also engulfs bacteria and cell debris. established physiology Source 8, 1, 3
   ⚖ When the balance tips

   too much — If many active forms live in the gut, they use up more nutrients from the gut contents and occupy more of the lining's surface.

   too little — If few live in the gut, they often go unnoticed; Blastocystis and Dientamoeba fragilis are also found in people without complaints, and their significance is disputed.

   contested · Source 5, 6, 1

3. Active form → Attachment Giardia holds on to the cell surface with an adhesive disc. Entamoeba binds to sugar chains of the mucus layer via a lectin. Cryptosporidium and Cyclospora enter the gut cell. This lets them resist the flow of the gut contents. Source 1, 2, 3↓ depletes Attachment holds the organisms against the flow of the gut contents. Where they attach or enter, the gut cells change; with Giardia a shortening of the brush border has been described. observed in studies Source 1, 2, 3
   ⚖ When the balance tips

   too much — If many organisms attach, the surface through which gut cells absorb nutrients shrinks; the cells then release more salt and water into the gut.

   too little — If few attach, the lining is barely changed, and the organisms are more easily carried away with the gut contents.

   observed in studies · Source 1, 2

4. Attachment → Multiplication In the gut, the single-celled organisms divide. Cryptosporidium and Cyclospora pass through several forms in succession, including a sexual stage. Division determines how many organisms the gut harbours. Source 2, 4↓ depletes Division determines how many organisms the gut harbours. In Cryptosporidium, some of the new oocysts already hatch in the gut and settle there again. established physiology Source 2, 4
   ⚖ When the balance tips

   too much — If the organisms divide quickly, their number in the gut rises, and more walled forms reach the stool.

   too little — If they divide slowly, for example when the immune defence keeps them in check, their number stays small, and few forms appear in the stool.

   established physiology · Source 2, 7

5. Multiplication → Forms in the stool Encystation Most of these organisms form a wall again and leave the body with the stool. A PCR test looks for sections of their DNA there and so shows which species is present. With the stool they reach the environment. Source 1, 4, 5↑ supplies The new walls carry the organism onward: with the stool they reach water and the environment. Cyclospora oocysts first have to mature there for some days before they can colonise a new host. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — If many walled forms are shed, the PCR test finds more DNA, and the signal appears earlier.

   too little — If few are shed, their DNA may remain below the detection limit of the test.

   established physiology · Source 1, 4, 5

6. Mucosa → Dendritic cells Dendritic cells pick up fragments of the organisms and present them to T cells in the lymphoid tissue of the gut wall. This starts the targeted response. Source 7↑ supplies Dendritic cells pass the information on to acquired immunity: they help determine which type of T cell arises and where it acts later. observed in studies Source 7
   ⚖ When the balance tips

   too much — If many dendritic cells present fragments, more T cells are activated, and the response in the gut wall becomes stronger.

   too little — If few present them, the targeted response stays weak, and the organisms remain longer in the gut.

   observed in studies · Source 7

7. Dendritic cells → T cells T cells coordinate the response: they release signalling molecules that attract further immune cells, and they prompt B cells to form antibodies. Without them, Giardia is not cleared in animal models. Source 7↑ supplies T cells, especially those that release IL-17, coordinate the defence in the gut. In animal models, mice without this response do not get rid of Giardia. observed in studies Source 7
   ⚖ When the balance tips

   too much — If the T cell response is strong, the organisms are cleared faster; at the same time many immune cells gather in the gut wall, and in animal models the brush border shortens.

   too little — If it is weak, less IgA is formed, and in animal models the organisms remain longer in the gut.

   observed in studies · Source 7

8. T cells → IgA in the gut B cells form antibodies of the IgA class. These are released through the gut wall into the mucus, where they bind to the surface of the organisms. In this way they are carried away with the gut contents. Source 7↑ supplies IgA binds the organisms in the mucus before they attach and so helps carry them away with the gut contents. The formation of IgA cells and their homing to the gut wall depend on retinoic acid and vitamin D. observed in studies Source 7, 9, 10
   ⚖ When the balance tips

   too much — If there is plenty of IgA in the mucus, fewer organisms attach, and more are washed away.

   too little — If little IgA has formed, the organisms attach to the lining more easily; in animal models Giardia remains longer in the gut without IgA.

   observed in studies · Source 7

Further stations

• Cysts, oocysts — resting forms with a wall Outside the body, these single-celled organisms survive inside a tough wall. As cysts or oocysts, they reach the mouth with water or food and pass on into the stomach. Just a few dormant forms are enough to settle. Source 1, 4↑ supplies The wall lets the dormant form survive outside the body for a long time, even in cold water; Cryptosporidium oocysts also resist chlorine. Just a few dormant forms are enough to settle in the gut. established physiology Source 1, 2
  ⚖ When the balance tips

  too much — If many dormant forms are swallowed, more organisms hatch, and colonisation starts with a larger number.

  too little — If few are swallowed, their number may already be enough to settle; whether this succeeds also depends on the defences in the gut.

  established physiology · Source 1, 2

• Mucosa — mucus and epithelial cells The top layer of the gut consists of mucus and tightly joined cells. These cells recognise foreign molecules at their surface and then release signalling molecules. Its mucus keeps organisms at a distance. Source 7↑ supplies The lining is the first barrier: mucus keeps organisms at a distance, tightly joined cells seal the wall, and their messengers attract immune cells. Its nitric oxide slows Giardia down. observed in studies Source 7
  ⚖ When the balance tips

  too much — If the cells release many messengers, more immune cells move into the gut wall, and mucus production increases.

  too little — If the cell junctions are loosened, as described for Giardia, more substances from the gut contents reach the wall.

  observed in studies · Source 7

Cofactors in this pathway

• Gastric acid (protons) — The acidic stomach content is one of the signals that opens the wall of the dormant form Source 1, 2
• Bile salts — The second signal for hatching in the small intestine, besides warmth and acid Source 1, 2In the ORY catalogue as a laboratory value: Gallensäuren (Stuhl)
• Cholesterol — Giardia does not make it itself; when the supply in the gut falls, cyst wall formation starts; plenty keeps it active Source 8
• Vitamin A — Retinoic acid is formed from it and makes gut B cells switch to the IgA class; IgA binds the organisms Source 9
• Vitamin D — The active form makes maturing B cells express the homing receptor CCR10; so they find their way to the gut wall Source 10In the ORY catalogue as a laboratory value: Vitamin D

Sources

1. Einarsson E, Ma'ayeh S, Svärd SG. An up-date on Giardia and giardiasis. Curr Opin Microbiol 2016 · PubMed 27501461
2. Tzipori S, Griffiths JK. Natural history and biology of Cryptosporidium parvum. Adv Parasitol 1998 · PubMed 9554069
3. Mann BJ. Structure and function of the Entamoeba histolytica Gal/GalNAc lectin. Int Rev Cytol 2002 · PubMed 12049210
4. Almeria S, Cinar HN, Dubey JP. Cyclospora cayetanensis and Cyclosporiasis: An Update. Microorganisms 2019 · PubMed 31487898
5. Stark D, Barratt J, Chan D et al. Dientamoeba fragilis, the Neglected Trichomonad of the Human Bowel. Clin Microbiol Rev 2016 · PubMed 27170141
6. Tan KS. New insights on classification, identification, and clinical relevance of Blastocystis spp. Clin Microbiol Rev 2008 · PubMed 18854485
7. Fink MY, Singer SM. The Intersection of Immune Responses, Microbiota, and Pathogenesis in Giardiasis. Trends Parasitol 2017 · PubMed 28830665
8. Luján HD, Mowatt MR, Byrd LG et al. Cholesterol starvation induces differentiation of the intestinal parasite Giardia lamblia. Proc Natl Acad Sci U S A 1996 · PubMed 8755526
9. Mora JR, von Andrian UH. Role of retinoic acid in the imprinting of gut-homing IgA-secreting cells. Semin Immunol 2009 · PubMed 18804386
10. 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

Whole pathway: Intestinal parasites

Related pathways

• Bacterial enteric organisms — attachment
• Clostridioides difficile — Gallensäuren (Stuhl), Vitamin D
• Microsporidia — Gallensäuren (Stuhl), Vitamin D
• Viral enteric organisms — Gallensäuren (Stuhl), Vitamin D

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