Lymphopenia: Causes, Consequences, and Clinical Approach

Definition

Lymphopenia = absolute lymphocyte count (ALC) <1.0 × 10⁹/L (adults). Lymphocytes (T, B, NK cells) are central to adaptive and innate immunity.

Major Etiologies

Category	Examples
Infections	HIV (CD4+ depletion), COVID-19, influenza, TB, sepsis
Malignancy	Leukemias, lymphomas, metastatic cancer (bone marrow infiltration)
Autoimmune	SLE, RA, Sjögren's (immune consumption, immunosuppressive Rx)
Iatrogenic	Corticosteroids, chemotherapy, radiation, biologics (rituximab, alemtuzumab)
Nutritional	Protein-energy malnutrition, zinc deficiency, B12/folate deficiency
Congenital	SCID, DiGeorge, ataxia-telangiectasia
Other	Splenectomy, chylous effusions, severe stress/surgery

Clinical Consequences

• Increased infection risk (opportunistic if CD4 <200)
• Impaired vaccine response
• Poor tumor surveillance
• May signal occult malignancy or HIV

Diagnostic Workup

1. CBC with differential → calculate ALC.
2. Flow cytometry (T/B/NK subsets) if persistent/unexplained.
3. HIV test, immunoglobulin levels, SPEP.
4. Review medications.
5. Consider bone marrow biopsy if pancytopenia or suspicion of infiltration.

Management

• Treat underlying cause.
• Prophylactic antimicrobials if severe/prolonged (e.g., TMP-SMX for PJP if CD4 <200).
• IVIG for humoral deficiency.
• Growth factors (G-CSF) not typically for lymphopenia alone.