A 32-year-old man from Delhi presents with a 3-month history of progressive weight loss (8 kg), chronic diarrhea, and oral candidiasis. He reports unprotected sexual contact 18 months ago. On examination, he has generalized lymphadenopathy and hepatosplenomegaly. CD4+ count is 85 cells/µL. A rapid HIV test is positive, confirmed by Western blot. Which of the following best explains the severe immunodeficiency seen in this patient?

Explanation

## Mechanism of HIV-Induced Immunodeficiency **Key Point:** HIV causes CD4+ T cell depletion through multiple mechanisms, with direct cytolytic infection and syncytia formation being the primary drivers of severe immunodeficiency. ### Direct Viral Mechanisms 1. **Direct infection and lysis**: HIV binds to CD4 and coreceptors (CCR5/CXCR4) on T cells, integrates into the genome, and produces viral particles that lyse the host cell upon budding. 2. **Syncytia formation**: In advanced disease, HIV envelope proteins (gp120/gp41) expressed on infected cells fuse with uninfected CD4+ T cells, creating multinucleated giant cells (syncytia) that undergo apoptosis. 3. **Chronic activation**: HIV triggers persistent immune activation, leading to increased T cell turnover and exhaustion. ### Why CD4+ Count Falls to <100 cells/µL With a CD4+ count of 85 cells/µL, this patient has AIDS (CD4 <200 cells/µL) with severe immunodeficiency. The combination of direct cytolysis and syncytia formation accounts for the rapid depletion seen in untreated infection. **High-Yield:** The CD4+ T cell count is the single best marker of HIV disease progression and predictor of opportunistic infection risk. At CD4 <100 cells/µL, risk of cytomegalovirus (CMV), *Cryptococcus neoformans*, and *Mycobacterium avium complex* (MAC) is very high. **Clinical Pearl:** Oral candidiasis and chronic diarrhea in this patient are classic signs of advanced HIV disease (AIDS-defining illnesses) that correlate with CD4 <100 cells/µL. ### Table: CD4+ Count Thresholds and Associated Opportunistic Infections | CD4+ Count | Opportunistic Infection Risk | | --- | --- | | 200–500 cells/µL | *Pneumocystis jirovecii* pneumonia (PCP), Tuberculosis | | 100–200 cells/µL | Toxoplasmosis, Cryptococcal meningitis | | <100 cells/µL | CMV, MAC, *Cryptococcus*, severe candidiasis | | <50 cells/µL | CMV retinitis, CMV colitis | [cite:Harrison 21e Ch 197]