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HIV - a global challenge   page 2
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2. Infection by HIV Link to the Medical Research Council web site
HIV structure
Figure 2. The structure of the human immunodeficiency virus - strains HIV-1 and HIV-2 have been identified.


Viral attack
Following infection with any virus for the first time, the various components of the immune system go on 'red alert'. Initially, the viruses inside susceptible cells multiply unchecked, but after a short time there is the first immune response. This is the T-cell response, made up of both 'killer' T-cells (CTLs) which destroy virus-infected cells, and 'helper' T-cells (CD4 T–cells). The helper cells secrete an array of chemicals known as cytokines. Some of these molecules are involved in the control of inflammation; others have antiviral activity, and others stimulate B cells to produce antibodies against the invading virus.
Enter HIV
Once infected by HIV a person is said to be HIV positive because they give a positive result to the test for antibodies to HIV. This condition generally persists throughout life. The sobering fact about HIV is that it primarily invades the very cells of the immune system which are designed to defend the body against attack.

Look at figure 3; it shows an HIV retrovirus about to infect a T–cell.

Present on the surface of helper T-cells and macrophages are receptor proteins, notably CD4. A glycoprotein on the surface of H|V, called gp120, binds to this receptor. Following fusion of the viral and CD4 T-cell membranes, HIV becomes enclosed within the cell. Here, the genetic material of the virus (RNA) is converted through the action of viral reverse transcriptase into DNA. This 'proviral DNA' then becomes integrated into the host cell genome.

This DNA remains latent until genetic material of the host CD4 T–cell is replicated or translated into proteins.

Animated diagram
Animated diagram Animated diagram
Figure 3. Life cycle of HIV in a CD4 T–cell.




Question 1
a) Explain why a virus may be regarded as a parasite.

b) The genetic information in HIV is in the form of ribonucleic acid (RNA). List three other types of virus which contain RNA.

c) What is the provirus and what happens to it once formed?

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