Life Cycle of HIV

The HIV infects and eliminates the important cells of immune system that results in the weakness of the body to defend itself from infections.

The simplified life cycle of HIV may be summarised in the following 4 steps:

1. Attachment: When HIV is introduced into the human blood stream, the virus circulates throughout the entire body but infects only CD4+ cells (special kind of WBCs). The HIV recognizes a specific kind of target cells because every kind of cell in the human body has specific cell-surface glycoprotein markers that serve to identify them. Each HIV possesses a glycoprotein called gp120 on its surface that precisely fits a cell surface marker protein called CD4 on the surfaces of immune system cells called macrophages and T-cells. Macrophages are infected first.
2. Entry into Macrophages: HIV requires a second macrophage receptor called CCR5. After gp120 binds to CD4, it grows to bind to CCR5. It forms gp120-CD4 complex into the cell through the cell membrane.
3. Replication: HIV has an enzyme called reverse transcriptase that synthesizes a double stranded DNA complementary to the virus RNA. This double stranded DNA directs the host cell machinery to produce many copies of the virus. Over a period of years HIV synthesises large number of viruses.
4. Entry into T-cells: The HIV constantly replicates and mutates. It causes the gp120 protein to change. Now it requires a different second receptor known as CXCR4 which occurs on the surface of T-lymphocytes CD4+ cells. Soon the body's T-lymphocytes becomes infected with HIV. New viruses rupture the cell membrane, killing the infected T-cells. It blocks the immune response leading to the onset of AIDS.