HIV is a virus belonging to the Retroviridae family and Lentivirus genre. There are two types of HIV: HIV-1 and HIV-2 which only have partial sequence homologies. HIV-2 is similar to SIV, the monkey AIDS virus, unlike HIV-1. Most seropositive people carry the HIV-1 type.
The viral particle is 150 nanometres in diameter and contains two copies of its 9,200 nucleotide long RNA, coding for 19 proteins. The viral particle itself is surrounded by a lipid membrane obtained from the host cell. Viral proteins are inserted into the membrane, in the same way as gp120 that is used as the anchor protein on the host cell.
AIDS viral transmission occurs through sexual contact, blood transfusion, injections (syringes), from mother to child across the placenta, and throughbreast-feeding. The viral particles contained in these body fluids recognise specific receptors (CD4 proteins) and co-receptors on the surface of the cells they infect (lymphocytes, macrophages, and dendritic or microglial cells).
HIV infection reduces the T4 lymphocyte count through three mechanisms. The virus kills the infected cell, increases the rate of cellular apoptosis and the CD8 lymphocytes recognise and kill infected cells.
The AIDS virus (in green) can infect immune system cells through their CD4 receptor. © DR