intraepithelial lymphocytes
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Immune sentinels embedded in the gut lining — their elevated count is the earliest detectable sign of celiac disease.
Intraepithelial lymphocytes are immune cells (predominantly T cells) that sit within the epithelial layer of the intestinal lining — physically embedded among the enterocytes rather than below them. They function as a first line of immune surveillance, patrolling for pathogens and abnormal cells.
Normal IEL density: fewer than 25 per 100 enterocytes. In celiac disease, this rises sharply — sometimes to 40–60 per 100 enterocytes or higher.
IELs and the Marsh Classification
Elevated IEL count is the earliest and most sensitive histological marker of celiac activity, appearing before crypt-hyperplasia or villous-atrophy:
- Marsh 1 — elevated IELs only (>25–40 per 100 enterocytes); villi and crypts still normal
- Marsh 2 — elevated IELs + crypt hyperplasia; villi still normal
- Marsh 3a–3c — elevated IELs + crypt hyperplasia + progressive villous atrophy
See marsh-classification for the full grading table.
Two Types: Normal vs Aberrant
In standard celiac disease, IELs have a normal phenotype — they're elevated in number but otherwise behave like normal T cells. This is the case in Marsh 1–3 and in NRCD.
In RCD type 2, IELs have an aberrant phenotype — they lose normal surface markers and gain abnormal ones, behaving more like cancer precursor cells. This distinction (normal vs aberrant IELs) is what differentiates RCD type 1 from type 2, and it carries major implications for prognosis and treatment. Aberrant IELs are driven by il-15 and carry a high risk of progression to eatl.
IELs Are Not Exclusive to Celiac
Elevated IELs can be caused by other conditions — ncgs, H. pylori infection, NSAIDs, autoimmune enteropathy, and others. This is why the Marsh classification requires clinical and serological context (TTG IgA, symptoms) to be diagnostically meaningful.
Related Concepts
enterocytes | marsh-classification | villous-atrophy | crypt-hyperplasia | refractory-celiac | il-15 | eatl