Why can the onset of AIDS take several years?

Answer 1

As you are probably aware, Acquired Immune Deficiency Syndrome is the disease caused by the Human Immunodeficiency Virus (HIV), and is characterized by the symptoms/effects of the virus. The common misconception is that the virus quickly attacks elements of the immune system, rendering it useless, and then opportunistic diseases set in, eventually leading to death. Instead, the body does quite a good job of fighting the virus. "Initially, HIV infection produces a mild disease that is self-limiting.....The patient experiences some mononucleosis-like symptoms (fever, rash, swollen lymph glands) but none of these are life-threatening. The result is an initial fall in the number of CD4+ cells and a rise in CD8+ cells but the numbers quickly return to near normal." "Cytotoxic B and T lymphocytes mount a strong defense and virus largely disappears from the circulation. During this period, more than 10 billion new HIV particles are produced each day but they are rapidly cleared by the immune system and have a half life of only 5-6 hours (some estimates show a half life of minutes). There can be up to 104 to 107 virus particles per ml of blood. (Titers of infectious virus are much lower indicating that much of the plasma virus is defective or neutralized). Most of this virus is coming from recently infected proliferating CD4+ cells. The infected cells that are producing this virus are destroyed either by the immune system or by the virus (half life about 1 day)." The long period of time that you are asking about is called 'clinical latency'. Although the immune system handles the virus for the most part, as described above, an infection is still usually present in other areas: "Nevertheless, the virus persists elsewhere, particularly in follicular dendritic cells in lymph nodes and here viral replication continues. Virus can become trapped in the follicular dendritic cell network of lymphoid tissue. The virus is also replicated by macrophages." So to directly answer your question, the reason for the long period of time before AIDS onset is that the HIV virus literally goes into recession due to the standard immune response system, but does does not die off completely due to it's effects on CD4+ cells and other immune factors. Eventually, these factors become fatal, however; this is explained further below. Some interesting figures that may be useful in quantifying how long this latency period actually is: "After the initial peak of virus, the virus reaches a "set point" during latency. This set point predicts the time of onset of clinical disease. With less than 1000 copies/ml of blood, disease will probably occur with a latency period of more than 10 years. With less than 200 copies/ml, disease does not appear to occur at all. Most patients with more than 100,000 copies per ml, lose their CD4+ cells more rapidly and progress to AIDS before 10 years. Most patients have between 10,000 and 100,000 copies per ml in the clinical latency phase " After this period of time, however, a few factors take effect that eventually produce AIDS. Responding CD4+ type cells become infected and tolerant of other infected cells. The number of these important immune-response cells then decreases dramatically. Eventually, the entire immune response is ceased, leading to oppertunistic infection. Check out the document linked below, especially section 1.4, for Most of the above quoted information came from: University of South Carolina, Dept. of Pathology and Microbiology. As for why AIDS antibodies are screened as opposed to the virus itself, this is mostly because of the goals of AIDS testing, which must be sure to not result in any false negatives (not detecting it) and must be relatively easy and efficient to perform, as AIDS testing is now quite common and inexpensive. Therefore, because directly detecting the actual virus is impractical at the scale needed, the results of the virus are screened for. One is to count the number of T4 cells present per volume of blood, which is an important indicator of the progress of the disease. The other method, detecting the presence of specific antibodies that are produced in response to the virus, " There are two primary tests used to detect antibodies to HIV - the ELISA and the Western Blot Assay. The ELISA, or enzyme linked immunosorbent assay, is used in most testing centers as an initial screening test (largely because it is inexpensive, has standardized procedures, and provides quick results). The problem with ELISA is that it is not 100% accurate. ELISA is very sensitive, thus the test recognizes even small amounts of HIV antibodies. The sensitivity is set extremely high because it is better to have some "false positives," than to miss the possibility of HIV infection. When you get tested, ELISA is performed as an initial test. If test results are positive, you will be re-tested with ELISA. If the second test also returns a positive diagnosis, the blood sample is tested using a Western Blot Assay. The Western Blot, unlike the ELISA, is very specific. It is both an expensive and time-consuming, labor-intensive test, thus it is only used for determining a "true-positive."

Answer 2

k bye