Question
Describe the structure of HIV. Outline the replication cycle of HIV inside a human host cell. Why does a decrease in CD4+ T-cell count below 200 cells/µL lead to AIDS?
(NEET 2022, similar pattern)
Solution — Step by Step
HIV (Human Immunodeficiency Virus) is a retrovirus with the following structure:
- Genetic material: Two copies of single-stranded RNA (ss-RNA) — it’s a diploid RNA virus
- Enzyme: Reverse transcriptase (converts RNA to DNA inside the host cell)
- Capsid: Protein coat (p24 protein) surrounding the RNA
- Envelope: Lipid bilayer (derived from host cell membrane) with glycoprotein spikes — gp120 (outer) and gp41 (transmembrane)
The gp120 protein is critical — it binds specifically to the CD4 receptor on helper T-cells.
- Attachment: gp120 on HIV binds to CD4 receptor on the helper T-cell surface
- Fusion and entry: The viral envelope fuses with the host membrane; RNA and reverse transcriptase enter the cell
- Reverse transcription: Reverse transcriptase converts viral ss-RNA into double-stranded DNA (ds-DNA)
- Integration: Viral DNA integrates into the host genome using integrase enzyme — now called a provirus
- Transcription and translation: The host cell machinery produces new viral RNA and viral proteins
- Assembly: New viral particles assemble near the host cell membrane
- Budding: New HIV particles bud off, taking a piece of host membrane as their envelope
The provirus can remain dormant for years — this is why HIV infection is lifelong.
CD4+ T-cells (helper T-cells) are the commanders of the immune system. They activate:
- B-cells (to produce antibodies)
- Cytotoxic T-cells (to kill infected cells)
- Macrophages (to enhance phagocytosis)
Normal CD4+ count: 500-1500 cells/µL
HIV specifically destroys CD4+ cells. As the count drops:
- Below 500: increased vulnerability to infections
- Below 200 cells/µL: AIDS is diagnosed. The immune system is so weakened that opportunistic infections (like Mycobacterium, Toxoplasma, Pneumocystis) and cancers (Kaposi’s sarcoma) take hold
Without CD4+ cells to coordinate the immune response, even normally harmless microbes become lethal.
Why This Works
HIV is uniquely devastating because it attacks the very cells that coordinate immune defence. It’s like taking out the generals of an army — even if soldiers (B-cells, killer T-cells) are present, they can’t mount an effective response without commands from helper T-cells.
The use of reverse transcriptase makes HIV a retrovirus — it flows genetic information from RNA to DNA (the reverse of the central dogma). Once integrated as a provirus, the viral DNA becomes a permanent part of the host genome. Every time the cell divides, it copies the viral DNA too. This is why we can’t cure HIV — we can’t remove the provirus from billions of infected cells.
Alternative Method — Transmission and Prevention Summary
For NEET, know the modes of HIV transmission: (1) unprotected sexual contact, (2) contaminated blood transfusion or shared needles, (3) mother to child (transplacental or through breast milk). HIV is NOT transmitted by casual contact, mosquito bites, or sharing food. NEET has asked “which of the following does NOT transmit HIV” — the answer is always the casual contact option.
Common Mistake
Students often write that “HIV is a DNA virus” — it’s an RNA virus (retrovirus). The DNA form exists only after reverse transcription inside the host. Also, don’t confuse HIV (the virus) with AIDS (the syndrome/disease). A person can be HIV-positive for years without having AIDS. AIDS is diagnosed only when CD4+ count drops below 200 cells/µL or when specific opportunistic infections appear.