Research updates
HIV - a global challenge   page 4
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3. The pattern of HIV infection Link to the Medical Research Council web site
 
HIV infection can be subdivided into different stages, although the duration of each stage is variable. Markers, such as the number of CD4 T–cells or number of viral particles, can give an indication of the current status of HIV infection.
Graph of infection
Figure 6. Pattern of HIV infection. The duration of each stage depends on the individual.
The stages of infection
Acute stage The appearance of HIV antibodies in the blood (‘seroconversion’) typically occurs after 3-12 weeks. Some newly infected individuals experience symptoms such as fever, sweats, head-ache, sore throats and lymph node enlargement.
Asymptomatic period
(latency phase)
This can last from 10 years (untreated HIV-1) to 15-20 years (HIV-2). Patients show no symptoms but are infectious to others. Recent research indicates an extremely high rate of replication of virus and infection of CD4 T-cells. However, new CD4 T-cells appear to replace those that are destroyed, leading to a steady state.
Symptomatic non-AIDS
(or pre-AIDS)
Patients suffer HIV related clinical symptoms, such as weight loss, malaise, fatigue, diarrhoea, night sweats and minor opportunistic infections such as oral thrush.
AIDS A decline in CD4 T–cell count and increase in circulating virus (viral load) indicates progression to AIDS. This is characterised by opportunistic infections and tumours (eg Kaposi’s sarcoma) as a result of severe damage to the cellular immune system. In addition, people with AIDS may develop severe weight loss (wasting) and/or dementia, the latter as a result of HIV infecting brain cells.
Photomicrograph of new virus particle
Figure 7. Photomicrograph of a new virus particle breaking free from the surface of a cell.
Living with AIDS - diagnosis and prognosis
A simple blood test for the presence of antibodies against HIV (‘HIV test’) is used to diagnose infection with the virus.

The blood of the person being tested is added to commercially prepared HIV proteins. Anti-HIV antibodies will bind to these proteins and the person is described as HIV positive.

Once diagnosed as HIV positive an individual will be offered routine blood tests. Clearly, the ability to predict the onset of AIDS would be of great advantage to patient and doctor. Currently, the results of two specific blood tests are used as markers to guide decisions about treatment. These are:

  • CD4 T-cell count. A normal count is generally considered to be 500 CD4 T–cells per millilitre of blood or above. Preventive treatment (prophylaxis) for common opportunistic infections is likely to be offered when the cell count drops below 200 because the risk of infection increases considerably at this point. Generally, the lower the CD4 T-cell count, the greater the risk of opportunistic infection.
  • Viral load test. This is usually measured as the number of cell-free virus particles (viral RNA molecules) in the blood.
 
Question 3
Look at Figure 6 and answer these questions.

a) Describe changes in levels of T cytotoxic cells and T helper cells (CD4 T-cells), antibodies and virus antigen throughout infection.

b) To what stage does the peak in virus antigen correspond?

c) Explain the relationship between killer T-cells (CTLs) and virus antigen.

d) Identify the latent stage. Why is this a misleading description?

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