Hi cynnryan,
Thank you for posing this quite interesting question! Many people are
familiar with the basic principles of an HIV infection, however the
long period of infection before AIDS onset is not really explained.
So let's do just that.
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. More general
information can be found in this factsheet from the National Institute
of Health:
[ http://www.thebody.com/niaid/stds/aids.html ]
The common misconception is that the virus quickly attacks elements of
the immune system, rendering it useless, and then oppertunistic
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 decreses 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
[ http://www.med.sc.edu:85/lecture/HIV3.htm ]
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." "
[ http://www.iapac.org/Text/hiv101final.asp#6 ]
I hope this information sufficiently answered your question. If not,
please request an answer clarification before rating this answer.
Thank you!
Sincerely,
Andrewxmp
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