Rare diseases · Sign or symptom
Decreased total T cell count
Low T cell count
HP:0005403
What it means
Abnormal decrease in the absolute number of T cells, commonly characterized as CD3+ lymphocytes, per microliter of blood, compared to a reference range for a given sex and age-group. These may include both TCR alpha/beta and gamma/delta T cells.
Rare diseases that can present with this25
Very common80–99%
8- Atypical/leaky severe combined immunodeficiency due to partial RAG defect
- Cernunnos-XLF deficiency
- Combined immunodeficiency due to DOCK8 deficiency
- Progeria-short stature-pigmented nevi syndrome
- T-B-NK+ severe combined immunodeficiency due to DCLRE1C deficiency
- T-B+NK+ severe combined immunodeficiency due to FOXN1 deficiency
- T-B+NK- severe combined immunodeficiency due to JAK3 deficiency
- T-cell immunodeficiency with thymic aplasia
Common30–79%
10- Hepatic veno-occlusive disease-immunodeficiency syndrome
- PGM3-CDG
- PGM3-CDG
- Skeletal dysplasia-T-cell immunodeficiency-developmental delay syndrome
- T-B-NK- severe combined immunodeficiency due to adenosine deaminase deficiency
- T-B+NK+ severe combined immunodeficiency due to CD3delta/CD3epsilon/CD3zeta deficiency
- T-B-NK+ severe combined immunodeficiency due to complete RAG1/2 deficiency
- T-B+NK- severe combined immunodeficiency due to gamma chain deficiency
- T-B+NK+ severe combined immunodeficiency due to IL-7Ralpha deficiency
- T-B+NK+ severe combined immunodeficiency due to IL-7Ralpha deficiency
The range is how often this sign appears among people diagnosed with each disease — it is not a probability of having any of them. Many of these signs also occur in common, non-rare conditions, and Orphanet separately records 1 disease where this sign is specifically absent. A sign on this page is a conversation to have with a clinician, not a conclusion.
Part of the broader category
Also called: Decrease in T cell count · Decreased absolute count of circulating T cells · Decreased CD3+ T cell proportion · T cell lymphopenia · T lymphocytopenia
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Start free with EleplanThis page uses the Human Phenotype Ontology (2026-06-23) — Human Phenotype Ontology Consortium, hpo.jax.org. Disease associations from Orphanet (INSERM), CC BY 4.0. Content is educational and is not medical advice.