In healthy adults, the number of T4 cells make up
between 32% and 68% of the total number of lymphocytes
a large group of white blood cells that include T4 cells
In fact, the lab uses the T4 percentage to determine
the number of T4 cells in a sample of blood.
The T4 percentage is sometimes
a more reliable measurement than the
T4 count because it tends to vary less
between measurements. For example, one
person's T4 count may vary between 200
and 300 over a several month period while
their T4 percentage remains constant at,
say, 21%. Provided that the T4 percentage
stays at 21% or higher, the immune system
still appears to be functioning properly,
regardless of what the T4 count is. At the
same time, a T4 percentage at or below 13% –
regardless of what the actual T4 count is –
usually means that the immune system is damaged
and that it is time to begin prophylactic treatment
(drugs to prevent diseases) for opportunistic
infections like PCP.
Imagine that you have all the letters of the alphabet
floating around inside your Tcells. When you are healthy,
you have lots of each letter. When your body is under attack
by a particular germ, virus, bacteria or parasite, the immune
system must "spell out" a particular word in order to combat it.
For example, let's say I had the onset of an infection.
In order to fight it, my immune system would need to
"spell out" the word "zebra." Each cd4/cd8 represents
a letter or "type" of fighter cell. If I didn't have
any "b's" left, then I wouldn't be able to fight the
infection with the same effectiveness, but it could
still pull through. Now if I was missing "b's" and "z's"
and was running low on "r's" then it gets almost impossible
for my body to fight off this infection. And if another
infection sets in that also requires any of those same
letters, then I could be in serious trouble.
When your T-Cells are low, and they climb back up, it's
a good thing. Absolutely. Tcells reproduce BUT they only
reproduce copies of themselves, of course. So, basically,
when you run out of "b's" you're out of "b's forever.
There are none there to reproduce and that "type" of figher
cell is now extinct in your system. But that's ok. If you
take care of yourself and stay on top of your health, you
can still avoid any nasty attacks on your immune system.
This is why it's called an "opportunistic infection."
If there are missing "letters" in your body's defense
system, then the "opportunity" is there for an infection
to progress and eventually overwhelm you and possibly kill you.
The more Tcells you preserve in the beginning, the more
of each letter you can hang onto.
When the immune system becomes suppressed,
some of the letters are lost. However, it's not
clear at what T-cell count -- less than 200??? --
that holes in the alphabet occur. It's also not clear
if this happens to everyone. For example, some people
with seriously suppressed T-cell counts may still
have enough (albeit a low number) of all the letters
to keep opportunistic infections at bay... at least
for a period of time. Obviously, people who lose
important letters run the greatest risk of becoming
sick (unfortunately, there are no laboratory tests
to check for all these letters).
Now, as for people who start treatment, it's true that
the letters that are still present begin multiplying.
However, research has determined that, once the virus
is put under control, the bone marrow begins turning
out "naive" T-cells... sort of like blank Scrabble
pieces. Provided that the virus remains under control,
those blank Scrabble pieces can be trained to respond
to certain germs that can invade the body... which means
that they adopt a letter that is missing from the alphabet.
And over time, these cells multiply... which further
helps keep people out of harms way. It can take several
months for all the letters to be replaced... and sometimes,
certain letters really are gone for good... but there's
plenty of evidence to suggest that antiretroviral treatment
is effective in bringing back letters that are necessary
to fight disease.