Understanding Immunodeficiency in AIDS: Why Infections Occur

AIDS (acquired immunodeficiency syndrome) is the advanced stage of HIV infection characterized by severe CD4+ T‑cell depletion (<200 cells/µL or opportunistic illness). This profound immune defect leads to susceptibility to opportunistic infections (OIs) and certain cancers.

Mechanisms of Immune Failure

1. CD4+ T‑cell loss: HIV infects and destroys these “helper” cells that coordinate both humoral (antibody) and cell‑mediated immunity.
2. Impaired antibody production: Without CD4+ help, B cells cannot class‑switch or generate high‑affinity antibodies.
3. Defective cell‑mediated immunity: CD8+ cytotoxic T cells and macrophages aren’t properly activated, failing to clear intracellular pathogens.
4. Ongoing viral replication: Further damages lymphoid tissue and drives immune activation/exhaustion.

Classic Opportunistic Infections

• Pneumocystis jirovecii pneumonia (PCP)
• Cryptococcal meningitis
• Toxoplasma encephalitis
• Mycobacterium avium complex (MAC)
• Cytomegalovirus (CMV) retinitis, colitis
• Progressive multifocal leukoencephalopathy (PML)
• Kaposi sarcoma (HHV‑8 associated)
• Severe candidiasis (esophageal, bronchial)

Prevention and Management

• Antiretroviral therapy (ART): Suppresses HIV, allows CD4 recovery—cornerstone of OI prevention.
• Prophylaxis: TMP‑SMX for PCP (CD4 <200), azithromycin for MAC (CD4 <50), fluconazole for candidiasis, etc.
• Vaccinations: Inactivated vaccines per guidelines; avoid live vaccines if severely immunocompromised.
• Regular monitoring: CD4 count, viral load, OI screening.

With effective ART, immune reconstitution dramatically reduces OI risk, allowing near‑normal life expectancy.