HIV stands for Human Immunodeficiency Virus. HIV is a retrovirus that infects several types of cells in our body, most importantly the CD4+ T Lymphocyte. The CD4+ T-cell is a white blood cell that is a major component of the human immune system that helps fight infection/disease and some types of cancer. By killing CD4+ T-cells, HIV progressively destroys the body’s ability to fight infection. HIV is the causative agent for AIDS (Acquired Immunodeficiency Syndrome).
Mechanism of Infection
Upon entering the body, the viral particle attaches to a CD4+ cell (host cell) and enters that cell. Once inside the host cell, the virus releases its genetic material, which is changed from RNA into DNA and then integrated into the DNA of the host cell. Once integrated, the host cell replicates the viral genetic material and creates new virus. Newly formed HIV virions, which are capable of infecting other CD4+ cells, are now released by the infected host cell. HIV replication actively continues following initial HIV infection and the rate of CD4+ cell destruction is progressive. It is important to note that HIV cannot replicate by itself, outside of living host cells.
The most common variant of HIV is HIV-1, which has led to the worldwide AIDS epidemic. HIV-1 is the cause for almost all of the cases of HIV in the United States.
A second HIV, known as HIV-2, has been discovered. Most cases of HIV-2 infection occur in West Africa, with HIV-2 infections appearing only sporadically in other parts of the world. HIV-2 cannot always be detected using the various laboratory tests for HIV-1 due to genetic variances in between HIV-1 and HIV-2.
HIV-2 infection differs from that of HIV-1 in that it usually has a longer latency period before the onset of AIDS and a lower viral load. It also is characterized by higher CD4+ lymphocyte counts than that of HIV-1 infection until clinical AIDS has developed and a less aggressive course of AIDS. Those with HIV-2 infection have a mortality rate that is 1/3 lower than HIV-1. It is important to note that HIV-2 infection in the United States is not common and is actually a rare event.
What is AIDS?/How does HIV cause AIDS?
AIDS (Acquired Immunodeficiency Syndrome) was first diagnosed in the U.S. in 1981. The term applies to the most advanced stages of HIV infection. For any HIV-infected individual, the probability of disease progression is most often difficult to predict. CD4+ T-lymphocyte levels are easily measured and can correlate with the relative risk of development of opportunistic diseases or death. The Centers for Disease Control revised its definition of AIDS in 1993 to include all HIV-infected people who have less than 200 CD4+ T-cells, or a CD4 percentage of less than 14%, whether the person has developed a true AIDS defining condition or not. (Healthy individuals, on average, have CD4+ T-cell counts of 1000 or more). A decrease in the total CD4+ T-cell count below 500 CD4+ T-cells/microliter of blood usually foretells the development of clinical AIDS, and a drop below 200 CD4+ T-cells/microliter of blood not only defines AIDS, but also indicates a high probability for the development of AIDS-related opportunistic infections and/or cancers. The definition also includes 26 clinical conditions that affect individuals with advanced HIV disease. Most of these AIDS-defining conditions are opportunistic infections- infections that normally do not cause harm in healthy individuals. However, due to the gradual destruction of the immune system in an HIV-infected individual, these infections take advantage of the weakened immune system, and are hence called opportunistic.
AIDS is distinguished by a very long latency period before the development of any visible signs of infection. The median time from initial infection to the development of AIDS is approximately 10 years, although the rate of disease progression varies substantially among patients. The average HIV infected individual may have an initial acute self-limited illness, take up to several weeks to seroconvert, and then may live up to 8-10 years on average before the development of the clinical signs and symptoms of AIDS.