What is HIV like?
Human Immunodeficiency Virus or HIV is one of the retroviruses. It has RNA as its genetic material. This is transcribed to DNA by the enzyme reverse transcriptase, which HIV contains together with the RNA.
HIV targets cells that form part of the immune system. Its main target is a type of cell called a CD4 T-lymphocyte. These cells are also called T-helper cells, because they ‘help’ other cells in the immune system to mount an immune response to pathogens in the body. Without this response, pathogenic micro-organisms can
multiply in the body and cause disease.
HIV HIV (human immunodefiiency virus) is the virus now known to cause AIDS
AIDS (acquired immune defiiency syndrome) is a disease that causes its victim’s
immune system to degenerate leaving them vulnerable to infectious diseases and some types of tumour
macrophage (also called a white blood cell) is a cell that surrounds and destroys
HIV has spikes on its surface, the heads of which are made from the glycoprotein known as gp120. This binds with CD4, a protein that protrudes from various types of human cell. A gp120 sticking out of an HIV virus particle connects with a CD4 sticking out of a cell like an egg fitting into an egg cup. Once the virus has attached to a cell, it can go on to the next stage and merge with the host cell.
Besides the T-helper cells, there are other types of cell that carry CD4 on their surface – such as macrophages and some natural killer cells. T-helper cells are the most important, though, because they are co-coordinators of the immune system. If their activity is impaired, it can have serious effects on the body’s response to
infections by other organisms.
How does HIV reproduce and cause AIDS?
After HIV has bound to the CD4 receptors on the surface of the T-helper cell, the following events occur:
1. It fuses with the plasma membrane and then releases its RNA and reverse transcriptase enzyme into the cell.
2. Th reverse transcriptase converts the RNA into DNA using building blocks called nucleotides, which are provided by the cell.
3. Th viral DNA becomes incorporated into the cell’s own DNA.
4. Th viral DNA is transcribed to viral RNA, which starts producing viral proteins, including the enzyme reverse transcriptase.
5. Th RNA, proteins and reverse transcriptase molecules are assembled by the cell into new HIV particles that escape by ‘budding’ from the cell membrane – this is an example of chronic release.
6. Th viruses then infect other T-helper cells.
Some HIV proteins remain on the surface of the infected CD4 cell and are recognized by the immune system – these cells are destroyed. Th cycle of infection, reproduction and destruction of infected cells repeats itself for as long as the body can keep replacing the CD4 lymphocytes.
Eventually, the body will not be able to replace these cells, and the number of free viruses in the blood will increase dramatically – HIV may infect other areas of the body, including the brain. Because of the drastic reduction in the number of T-helper cells, the immune function is severely reduced and many opportunistic
infections may occur (including pneumonia and tuberculosis), together with rare cancers like Kaposi’s sarcoma.
Th period when the body keeps replacing the CD4 lymphocytes as fast as they are destroyed is called the latency period and can last for many years.
Can AIDS be treated?
Although there is no cure for AIDS and, as yet, no vaccine to give immunity against infection, there are a number of drugs – called anti-retroviral drugs – that can be effective in slowing down the progression to AIDS. These drugs work by blocking the reproduction of the virus in the CD4 lymphocytes. There are several
different drugs that act in different ways at different stages of the
cycle of reproduction.
Because the drugs act on different stages of the HIV life cycle, the most effective treatment is obtained when they are used together. This is called High Activity Anti-Retroviral Treatment (HAART). Although it is effective against HIV, it does have unpleasant side effects.
As well as infected cells being destroyed by natural killer cells, other lymphocytes make antibodies that target any free HIV in the blood. Th presence of these antibodies can be detected, and the person is diagnosed as being HIV-positive.