HIV Virology
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Direct answer
The human immunodeficiency viruses, HIV-1 and HIV-2, are enveloped lentiviruses carrying two copies of positive-sense ribonucleic acid with reverse transcriptase, integrase and protease. The surface glycoprotein gp120 binds the CD4 molecule of helper T lymphocytes with the chemokine co-receptors CCR5 or CXCR4, the genome is reverse-transcribed and integrated into host chromosomes, and progressive CD4 loss produces acquired immunodeficiency syndrome with opportunistic infections. Diagnosis is by fourth-generation antigen–antibody assays confirmed by supplementary tests, and treatment is lifelong combination antiretroviral therapy, dolutegravir-based in India.
What you must remember
- Structure: envelope with gp120 surface and gp41 transmembrane glycoproteins, cone-shaped p24 capsid — the core antigen of combo assays — p17 matrix, and two ribonucleic acid strands with reverse transcriptase, integrase and protease.
- Entry and tropism: gp120 binds CD4 and then CCR5 on macrophage-tropic strains or CXCR4 on T-cell-tropic strains; persons homozygous for the CCR5-delta32 deletion are relatively resistant.
- Replication: attachment, fusion, reverse transcription to proviral deoxyribonucleic acid, integration, transcription and translation, budding and protease-mediated maturation — each step a drug target.
- Transmission and course: sexual contact, blood and needles, and mother-to-child transmission in utero, at delivery or through breast milk; acute retroviral syndrome is followed by years of latency, then AIDS when CD4 counts fall below two hundred cells per microlitre or defining illness appears.
- Diagnosis: fourth-generation assays detecting p24 antigen and antibody shorten the window to roughly three weeks, nucleic acid tests detect earliest, and supplementary assays confirm; infants are tested by deoxyribonucleic acid polymerase chain reaction because maternal antibody confounds serology.
- Treatment: lifelong combination therapy, the first line in India being dolutegravir with tenofovir and lamivudine; post-exposure prophylaxis runs for twenty-eight days.
- Opportunistic infections: tuberculosis is the commonest and deadliest in Indian patients; others are candidiasis, cryptococcal meningitis, Pneumocystis pneumonia, toxoplasmosis, cytomegalovirus disease and cryptosporidiosis.
Common confusion
The window period is presented as fixed, but it depends on the assay — nucleic acid tests detect earliest, combo assays by about three weeks, antibody-only assays later — so recent exposure needs repeat testing. HIV-2, present in India, is intrinsically resistant to non-nucleoside reverse transcriptase inhibitors, which matters when choosing regimens. A positive enzyme immunoassay alone never diagnoses HIV; confirmatory testing precedes informing the patient.
Exam-focused takeaway
In theory, draw and label the virion, write the replication cycle naming the enzyme at each step, then cover transmission, CD4 depletion, diagnosis and antiretroviral classes. In viva, expect the role of gp120, why the virus is called retro, the monitoring tests and prophylaxis duration. At the practical, explaining the rapid test algorithm with confirmatory testing and counselling is examined as much as the science.
Frequently asked questions
Why is HIV called a retrovirus?
Its reverse transcriptase makes deoxyribonucleic acid from a ribonucleic acid template — the reverse of the usual direction — and the provirus then integrates into the host genome.
Which cells does HIV infect?
CD4-bearing cells — helper T lymphocytes, macrophages and dendritic cells — using gp120 for CD4 binding and CCR5 or CXCR4 as co-receptors.
What is the window period?
The interval between infection and test positivity; fourth-generation assays narrow it to about three weeks, and nucleic acid tests detect infection even earlier.
How is HIV diagnosed in infants?
By deoxyribonucleic acid polymerase chain reaction from about six weeks of age, since passively transferred maternal antibody makes serology unreliable in infancy.
What is the first-line therapy in India?
A dolutegravir-based regimen, typically tenofovir with lamivudine and dolutegravir as one daily tablet, continued lifelong with viral load monitoring.
Why is tuberculosis emphasised in Indian HIV practice?
Tuberculosis is the commonest opportunistic infection and cause of death in Indian patients, can occur at any CD4 count, and all patients are screened for both.