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Paper II — 2014 March (2010 Scheme) — Q11

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Paper II
2014 March (2010 Scheme) · 40 marks · 120 min

Question

Significance of p24 antigen.

Q112 marksShort Notes

Answer

p24 antigen is a structural core (capsid) protein of HIV, encoded by the viral gag gene, and its detection in blood serves as a valuable early marker of HIV infection, complementing antibody-based testing.

Significance/timing: p24 antigen appears in the blood relatively early after infection — typically within 2–3 weeks, becoming detectable before HIV antibodies develop (antibody seroconversion typically takes 3–12 weeks, sometimes longer) — this earlier appearance makes p24 antigen detection particularly valuable for narrowing the “window period” (the interval between infection and detectability by a given test), during which a person is infected and potentially infectious but would test falsely negative on an antibody-only assay.

Clinical use:

  • 4th-generation combination HIV screening assays (combined antigen/antibody ELISA) simultaneously detect both p24 antigen and HIV antibody, providing improved sensitivity during early/acute infection compared to antibody-only (3rd-generation) assays, and are now the recommended standard screening approach in most modern testing algorithms.
  • p24 antigen levels typically rise during acute infection, then fall (as antibody develops and forms immune complexes with the antigen, reducing free antigen detectable by standard assays) during the chronic/latent phase, and can rise again in advanced disease/AIDS as immune control of viral replication deteriorates — making p24 antigen trend a rough surrogate marker of disease progression in some contexts, though it has been largely superseded by viral load (HIV RNA) testing for this purpose.
  • Useful in diagnosing acute (early) HIV infection, where a person may present with a non-specific febrile “seroconversion illness” but antibody testing is still negative.

Limitation: p24 antigen alone is less sensitive than viral RNA PCR for very early detection, and antigen levels can fall below detectable limits once antibody-antigen immune complexes form, making it primarily useful as a component of combination assays rather than a standalone test.

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