Question
HIV destroys the immune system by targeting which type of cells? Explain the sequence of events that leads from HIV infection to full-blown AIDS.
This is a high-weightage conceptual question. NEET 2023 asked it directly, and CBSE boards love asking the mechanism in 3-5 marks.
Solution — Step by Step
HIV (Human Immunodeficiency Virus) is a retrovirus that specifically attacks helper T-lymphocytes (T₄ or CD4⁺ cells). These cells act as the “commanders” of your immune army — they activate B cells, cytotoxic T cells, and macrophages. HIV docks onto the CD4 receptor on the surface of these cells and injects its RNA genome inside.
Once inside, HIV uses an enzyme called reverse transcriptase to convert its RNA into DNA. This viral DNA then integrates into the host cell’s own chromosome. Why does this matter? Because now the virus is invisible to the immune system — it’s hiding inside the very cells meant to fight it.
The infected helper T cell starts producing thousands of new HIV particles, which bud out and infect more T₄ cells. Eventually, the host cell is destroyed in the process. Over months to years, the CD4⁺ count drops steadily — from a healthy ~1000 cells/mm³ down below 200 cells/mm³.
With helper T cells decimated, the entire adaptive immune response collapses. B cells can’t make antibodies effectively. Cytotoxic T cells lose activation signals. The body can no longer mount responses against pathogens it would normally handle easily.
This immunodeficient state is called AIDS (Acquired Immunodeficiency Syndrome). The patient now falls prey to opportunistic infections — diseases caused by organisms that a healthy immune system would neutralise without trouble. Common ones: Pneumocystis jirovecii pneumonia, oral candidiasis (fungal), toxoplasmosis, CMV retinitis, and certain cancers like Kaposi’s sarcoma.
Why This Works
The reason HIV is so devastatingly effective is its target choice. Helper T cells are not just one type of immune response — they coordinate everything. Knocking them out is like cutting all communication lines in an army simultaneously.
The retroviral lifecycle (RNA → DNA via reverse transcriptase → integration as provirus) is also what makes HIV hard to eliminate. The viral DNA lies dormant in memory T cells, invisible to antiviral drugs that only act on actively replicating virus.
The window period (2–8 weeks post-infection) is when the virus replicates massively before the immune system catches up. ELISA tests during this window may give false negatives — this is why HIV testing recommends a repeat test after 3 months.
Alternative Method (Answering as a Flow Diagram)
For NEET and CBSE 5-mark answers, a labelled flow is often the cleanest approach:
HIV enters body (blood/sexual contact/vertical transmission)
↓
Binds CD4⁺ receptor on Helper T cells
↓
Reverse transcriptase: viral RNA → viral DNA
↓
Viral DNA integrates into host chromosome (provirus)
↓
Host cell destroyed → new virions released
↓
CD4⁺ count falls below 200 cells/mm³
↓
Adaptive immunity collapses
↓
Opportunistic infections → AIDSDrawing this in an answer with labels for “reverse transcriptase” and “CD4⁺ cells” fetches full marks.
NEET frequently asks: “Which enzyme does HIV use to replicate?” — Answer: Reverse transcriptase. Also remember, HIV is a retrovirus because it reverses the central dogma (RNA → DNA instead of DNA → RNA).
Common Mistake
Students write “HIV destroys all WBCs” — this is wrong and will cost marks. HIV specifically targets helper T lymphocytes (CD4⁺ T cells). Other WBCs like neutrophils, B cells, and cytotoxic T cells are not directly killed by HIV — they fail indirectly because they lose the activation signals that helper T cells normally provide. Be specific in your answer.
A second common slip: confusing HIV (the virus) with AIDS (the syndrome/disease state). HIV infection does not immediately mean AIDS. A person can carry HIV for years with a normal CD4⁺ count before progressing to AIDS. The NEET question specifically distinguishes these.