Mature T

Mature T
I believe that the mature T lymphocytes may be
out in various tissues in the lymphatic system. I
had occasionally wondered about this myself, since
back in grad school I learned that the thymus is
replaced by connective tissue after puberty.
However, after more research, I think the truth is
that while the active tissue in the thymus shrinks
as we age, the loss of the epithelium (where most
of the action is) happens so slowly that few
people live long enough to completely outlast it.
Of course, we do get decreased immune function
with age.
This makes evolutionary sense. We
get fewer new T-cells and lessdifferentiation of
cells as we age, but it's less important then.
After all, we are born with all of the variety of
T-cells we'll ever have. Exposure just allows us
to make active and memory cells for each clone.
Back when we lived in small groups, rarely went
very far, and rarely contacted anything or anyone
from very far away, how many new pathogens were we
going to see after age 15? Thymus tissue may be
expensive to support, so why keep it after you had
produced your lifelong arsenal? Besides, people
were probably lucky to live to age 40.
Now
that we may live to past 90 and spend our lives
jetting around the globe and eating food grow on
another continent, it would be great to still have
an active thymus, but that's a problem with
evolution, it can't look ahead to see what you'll
need in the future.
As you may know, T-cell progenitor cells (stem
cells) originating in bone marrow migrate to the
thymus where they undergo a remarkable maturation
process.Here, the immature T-cells (thymocytes)
differentiate into different classes, including
T-helper cells and cytotoxic T-cells. Afterwards,
the cells undergo a critical selection process in
which only T-cells capable of recognizing non-self
antigens displayed on the surface of
"professional" antigen-presenting cells (e.g.
macrophages, dendritic cells) are allowed to
survive. As much as 98% of the T-cells fail this
selection process and are degraded. The rest
migrate to peripheral sites throughout the body to
perform their specific immunological roles. To
see animation illustrating this process in more
detail, go to
The
immune system's library of T-cells is largely
built early on in life. A stockpile of memory
T-cells is created (generated by previous exposure
to foreign antigens), helping to prepare your body
for future infections. The thymus begins to
shrink after puberty and its capacity to produce
immune cells gradually reduces, but may not
completely diminish. Also, while the majority of
T-cells mature in the thymus, there have been
reports of T-cell maturation in the liver and
intestines. This means that if an older adult is
exposed to a unique antigen he/she has never
encountered before, a T-cell response is still
possible. However, this response may not be as
efficient as it would be in a child. With this in
mind, it seems then that the philosophy of keeping
our kids as clean and germ-free as possible may
actually limit their ability to build a strong,
healthy immune system!
Where are mature T lymphocytes after disappear
the thymus?

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Home NEWS OF NOTE Mature tree preservation secrets

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T he tree was absolutely massive. I had never measured it, but it was easily 20 feet in circumference and 120+ feet high. Its limbs stretched to the sky as if to support the clouds. If that wasn’t enough, it stood at the top of a hill overlooking a heart-stopping view to the west. From the hilltop, you could see for miles down a pristine valley of open fields and forest. Turn around, and you faced the tree. You were dwarfed by its size, feeling like a single leaf at the tip of a branch as thick as your body.
I lived nearby, so I went to the hill often to admire the view and marvel at that tree. I was always undecided by which way to stare. The view to the west is easily one of Virginia’s finest, but it was the tree that stirred my thoughts most. What had it witnessed over the course of its long life? What had happened under its spreading branches, and why? Civil war lookouts? Marketplaces? Hunters? Lovers? Duels? Hangings? While my imagination ran rampant, the tree stood silent, filled with a knowledge I would never know.
Occasionally, I would invite friends to ride horseback there to stare, marvel and muse. Eventually, I dropped off a small picnic bench, a place to sit, look and think. Sometimes, I would pack a meal, blankets to spread, and tiki lamps for night light. Guests would reverently watch the summer sunset as we shared thoughts about the history that tree had witnessed.
Today, I would consider the massive amount of carbon stored in that trunk. Imagine the work this tree had done for us all! While traffic streamed east and west on a nearby 4-lane highway, the tree was quietly pulling carbon emissions from the air to be stored in its trunk as it grew larger and larger. The genius of nature is amazing when you consider its patience, persistence and beauty in the process of healing our dirty work.
Nothing lasts forever, though. Even the proudest of trees eventually die. In this case, weather took a toll. Large limbs came crashing down as pieces of the tree began to succumb. Then, buyers found the spectacular view and bought the site to build their forever home. In the construction process, the tree was removed.
It was a sad day for me. But now, years later, I feel profound hope. As it turns out, the new landowners heard about our work to grow trees for carbon and nutrient credits, became interested and are now committed. Their goal will be to convert 40 acres of pasture at the base of the hill into a hardwood forest. The same trees will do double duty cleaning the air and water. Two programs. Two ecological benefits. Two financial rewards, as well. It’s no secret. You can do the same. Today, conservation means commerce in the Virginia nutrient trading program and the international carbon offset markets. As our founder, Chandler Van Voorhis, says, “By growing trees for today’s ecological credits, we give conservation a purpose and capitalism a heart.”
The landowner’s goal is laudable, to be sure. Replace one massive tree with 24,000 saplings on 40 acres! Yes, this gives me great hope, but you know, we need so much more. Yesterday, The Washington Post reported global warming is advancing far faster than projected. The World Meteorological Organization reports we will likely reach thresholds we were projecting to reach this century … in the next 5 years!
Pick up the May issue of National Geographic dedicated to saving forests. “Each year, forests and other vegetation absorb up to a third of the CO2 released from burning fossil fuel.” Yet, “Earth has lost a third of its forests over the last 10,000 years, half of that just since 1900,” notes senior environment writer Craig Welch. Today, extreme conditions related to climate change fuel the fires that multiply that massive loss many times over. What are we doing to replace the “lungs” of Earth? Sadly, not enough.
I may never live to see the saplings we manage grow to the size of my large limbed friend. But even when that happens, long after we are all gone, future generations dealing with seething oceans, scorched ground, hurricanes, tornados, and other extreme weather events will be asking, “Didn’t they see what was coming? Why didn’t they do more?” Believe me, now is the time to plant more trees.
Robert Banner is Senior Project Officer at ACRE Investment Management in The Plains, VA managing ecological credits for all concerned landowners. Contact Rob at rob@acre-investment.com, or call his cell (540) 729-1335.

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