Clinical meaning
Human immunodeficiency virus (HIV) is a retrovirus that selectively targets CD4+ T-helper lymphocytes, the central coordinators of the adaptive immune response. HIV uses the gp120 envelope protein to bind to the CD4 receptor and CCR5/CXCR4 co-receptors, then fuses with the host cell membrane. Reverse transcriptase converts viral RNA to DNA, which integrase inserts into the host genome as a provirus. Viral replication destroys CD4+ cells through direct lysis, syncytia formation, and immune-mediated apoptosis. Untreated HIV progressively depletes CD4+ cells from normal (500-1,500 cells/mm³) to < 200 cells/mm³, defining AIDS. The acute retroviral syndrome occurs 2-4 weeks after infection with flu-like symptoms and high viral load. The clinical latency period (asymptomatic phase) lasts years but viral replication continues. Opportunistic infections emerge as CD4+ count declines: oral thrush at < 400, Pneumocystis pneumonia at < 200, Mycobacterium avium complex and CMV at < 50.
Exam relevance
Risk factors: - Unprotected sexual contact with HIV-positive partner - Sharing contaminated needles for IV drug use - Perinatal transmission from untreated HIV-positive mother - Occupational needlestick with HIV-positive source - Receipt of contaminated blood products (rare in screened supply)