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
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.
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.Thanks for asking.
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 inmunology
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!
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