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lymphopenia

Introduction

peripheral blood lymphocyte counts below 1,500/µL
lymphocytes include T cells, B cells (which produce immunoglobulin) and CD8+ Killer Cells - see adaptive immunity

is associated with increased severity of viral infections and need for hospitalisations (eg. COVID-19 coronavirus (2019-nCoV / SARS-CoV-2) )
most commonly is due to viral infections but also occurs with many states which cause either:
- neutrophilia
  - such as acute stress response (including exercise), corticosteroids, inflammation including bacterial infections, rheumatoid arthritis, systemic lupus erythematosus (SLE), etc.
- neutropenia
  - impaired bone marrow functional capacity such as radiation injury, cytotoxic agents, leukaemia, lymphoma, aplastic anaemia
a low T cell count may indicate HIV / AIDS

2-4% of adults have levels chronically below 1,000/µL while 20% have levels chronically below 1,500/µL
this may result in impaired immune surveillance: fewer cytotoxic T cells and NK cells to eradicate nascent malignant cells, compounded by restricted T-cell receptor repertoire diversity from homeostatic proliferation. ¹⁾
appears to be independently linked to all-cause, cancer, and cardiovascular death with ~60% increased all cause mortality over a 9 yr 2020 Copenhagen study ²⁾
appears to be associated with increased risk of hospitalization with infection and increased risk of infection-related death in the general population ³⁾
may be treatable with an IL-15 agonist or increased resistance muscle training which releases IL-15 - both require further study

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