An introduction to the biology of HIV.
Human Immunodeficiency Virus (HIV) is the agent which causes the syndrome known as AIDS (Acquired Immune Deficiency Syndrome). HIV is spread through bodily fluids. Specifically HIV can be spread through sexual transmission, blood transfusions, and the sharing of hypodermic needles. As well, mothers can pass HIV on to their children at birth or through breast milk. In developed countries, an increased screening of blood products has reduced the transmission of HIV through blood transfusions.
HIV is a retrovirus of the lentivirus family. Retroviruses contain RNA which is transcribed into DNA and then integrated into the host chromosome. RNA is transcribed from the viral DNA incorporated into the host DNA. Viral proteins are translated from this RNA and used to make packaging material for viral particles. The RNA also serves as the genomic RNA for the new virions. Newly formed viral particles leave the host cell by budding from the plasma membrane which results in the formation of a membrane envelope.
Receptors present on host cells allow HIV to bind and enter the cell. Host cells expressing CD4 bind to glycoproteins gp120 and gp41 which are present on HIV. Because CD4 T cells, macrophages, and dendritic cells express CD4, HIV preferentially effects these cells leading to an impairment in the immune system.
CD4 T cells are integral to acquired immunity as they play an important role in antigen presentation that leads to the activation of B cells. They also induce the activation of macrophages. Because of this impairment in acquired immunity, AIDS sufferers often show a vulnerability to illnesses rarely seen in the average population. When AIDS was first noticed, it came to the medical fields’ attention in the form of Kaposi’s sarcoma in a young, male population. Kaposi’s sarcoma, a rare cancer of endothelial cells, was previously only seen in elderly, immunocompromised men.
