- WHAT ARE T-CELLS?
- WHY ARE T-CELLS IMPORTANT IN HIV?
- HOW ARE THE TEST RESULTS REPORTED?
- WHAT DO THE NUMBERS MEAN?
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T-cells are a type of lymphocyte (white blood cell). They are
an important part of the immune system. There are two main types
of T-cells. T-4 cells, also called CD4+, are “helper”
cells. They lead the attack against infections. T-8 cells,
(CD8+), are “suppressor” cells that end the immune response.
CD8+ cells can also be “killer” cells that kill cancer
cells and cells infected with a virus.
Researchers can tell the T-cells apart by specific proteins
on the cell surface. A T-4 cell is a T-cell with CD4 molecules
on its surface. This type of T-cell is also called “CD4 positive”,
or CD4+.
IMPORTANT IN HIV?
When HIV infects humans, the cells it infects most often are
CD4+ cells. The virus becomes part of the cells, and when they
multiply to fight an infection, they also make more copies of
HIV.
When someone is infected with HIV for a long time, the number
of CD4+ cells they have (their T-cell count) goes down. This is
a sign that the immune system is being weakened. The lower the
T-cell count, the more likely the person will get sick.
There are millions of different families of T-cells. Each family
is designed to fight a specific type of germ. When HIV reduces
the number of T-cells, some of these families can be totally wiped
out. You can lose the ability to fight off the particular germs
those families were designed for. If this happens, you might develop
(See Fact Sheet 500).
RESULTS REPORTED?
T-cell tests are normally reported as the number of cells in
a cubic millimeter of blood, or mm3. There is some disagreement
about the normal range for T-cell counts, but normal CD4+ counts
are between 500 and 1600, and CD8+ counts are between 375 and
1100. CD4+ counts drop dramatically in people with HIV, in some
cases down to zero.
The ratio of CD4+ cells to CD8+ cells is often reported. This
is calculated by dividing the CD4+ value by the CD8+ value. In
healthy people, this ratio is between 0.9 and 1.9, meaning that
there are about 1 to 2 CD4+ cells for every CD8+ cell. In people
with HIV infection, this ratio drops dramatically, meaning that
there are many times more CD8+ cells than CD4+ cells.
The T-cell value bounces around a lot. Time of day, fatigue,
and stress can affect the test results. It’s best to have blood
drawn at the same time of day for each T-cell test, and to use
the same laboratory.
Infections can have a large impact on T-cell counts. When your
body fights an infection, the number of white blood cells (lymphocytes)
goes up. CD4+ and CD8+ counts go up, too. Vaccinations can cause
the same effects. Don’t check your T-cells until a couple of weeks
after you recover from an infection, or after you get a vaccination.
Because the T-cell counts are so variable, some doctors prefer
to look at the T-cell percentages. These percentages refer to
total lymphocytes. If your test reports CD4+% = 34%, that means
that 34% of your lymphocytes were CD4+ cells. This percentage
is more stable than the number of T cells. The normal range is
between 20% and 40%. A CD4+ percentage below 14% indicates serious
immune damage. It is a sign of AIDS in people with HIV infection.
MEAN?
The meaning of CD8+ cell counts is not clear, but it is being
studied.
The CD4+ cell count is a key measure of the health of the immune
system. The lower the count, the greater damage HIV has done.
Anyone who has less than 200 CD4+ cells, or a CD4+ percentage
less than 14%, is considered to have AIDS according to the US
Centers for Disease Control.
CD4+ counts are used together with the viral load to estimate
how long someone will stay healthy. See Fact
Sheet 413 for more information on the viral
load test.
CD4+ counts are also used to indicate when to start certain
types of drug therapy:
When to start antiviral therapy:
When the CD4+ count goes below 350, most doctors begin antiviral
treatment (see Fact Sheet
410). Also, some doctors use the CD4+% going below 15% as
a sign to start aggressive antiviral therapy, even if the CD4+
count is high. More conservative doctors might wait until the
CD4+ count drops to near 200 before starting treatment.
When to start drugs to prevent opportunistic
infections:
Most doctors prescribe drugs to prevent opportunistic
infections at the following CD4+ levels:
- Less than 200: pneumocystis
carinii pneumonia (PCP)
- Less than 100: toxoplasmosis
and cryptococcosis
- Less than 75: mycobacterium
avium complex (MAC)
.
Because they are such an important indicator of the strength
of the immune system, official treatment guideline in the US suggest
that CD4+ counts be monitored every 3 to 6 months. See Fact
Sheet 411 for more information on the treatment
guidelines.
Revised November 16, 2002
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New Mexico AIDS Education and Training Center
and the New Mexico Department of Health