Microbiology Cheat Sheet
Hiv: Entry, Testing & Opportunists
HIV entry, testing and opportunists for NEET-PG: gp120 CD4 binding, p24 window period, NACO testing, CD4 thresholds, PCP, and cryptococcal meningitis.
MedNext Academy | 3 min read
Hiv: Entry, Testing & Opportunists
HIV entry, testing and opportunists for NEET-PG: gp120 CD4 binding, p24 window period, NACO testing, CD4 thresholds, PCP, and cryptococcal meningitis.
This topic covers HIV virology and entry, the NACO testing strategy, natural history, and the CD4-correlated opportunistic infections that define AIDS.
High-yield lines
- HIV gp120 binds the CD4 receptor while gp41 mediates membrane fusion, with CCR5 used by early macrophage-tropic strains and CXCR4 by late T-cell-tropic strains.
- The p24 capsid antigen appears before antibodies, and fourth-generation assays detect both p24 and antibodies, shortening the window period to about 2 weeks.
- Individuals homozygous for the CCR5-delta32 deletion are highly resistant to HIV-1 infection.
- Reverse transcriptase lacks proofreading, introducing about one mutation per cycle and driving antigenic variation and drug resistance.
- Integrase inserts proviral DNA into the host genome, establishing a latent reservoir in resting memory CD4 T cells that current therapy cannot eliminate.
- India's NACO strategy uses three sequential rapid tests based on different antigen principles for diagnosis in adults.
- In infants under 18 months, maternal IgG confounds serology, so diagnosis requires HIV DNA or RNA PCR.
- HIV RNA viral load is the best marker of antiretroviral therapy efficacy, while CD4 count guides opportunistic infection prophylaxis.
- Tuberculosis is the most common opportunistic infection and leading cause of death in HIV-positive individuals in India.
- Pneumocystis jirovecii pneumonia occurs when CD4 falls below 200 and is diagnosed by silver (GMS) staining of bronchoalveolar lavage.
- Cotrimoxazole prophylaxis is started at CD4 below 200 to prevent PCP, toxoplasmosis, and isosporiasis.
- Cryptococcal meningitis occurs below CD4 100, diagnosed by India ink and cryptococcal antigen (CrAg), with CrAg screening recommended below CD4 100.
- Cerebral toxoplasmosis and progressive multifocal leukoencephalopathy occur around CD4 50 to 100, while CMV retinitis and disseminated MAC occur below 50.
- Immune reconstitution inflammatory syndrome is a paradoxical deterioration after starting antiretroviral therapy, most common with TB and cryptococcal infection.
Mapped competency codes
- MI2.7
Continue into the full chapter
This summary maps to MI2-cvs-and-blood.
Frequently Asked Questions
Which HIV proteins mediate attachment and fusion?
gp120 binds the CD4 receptor and co-receptors, while gp41 mediates fusion of the viral and host cell membranes.
How is HIV diagnosed in infants under 18 months?
By HIV DNA or RNA PCR, because maternal IgG antibodies cross the placenta and confound antibody-based serological tests.
At what CD4 count is Pneumocystis prophylaxis started?
When CD4 falls below 200 cells per microlitre, using cotrimoxazole, which also covers toxoplasmosis and isosporiasis.
What is the most common opportunistic infection in HIV in India?
Tuberculosis, which is also the leading cause of death, diagnosed using GeneXpert MTB/RIF as the initial test.
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