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Non
encapsulated lymphoid tissue
There are accumulations of immune cells into regions that are
far less organized. These accumulations are not contained within
any form of capsule and they do not have sinuses running through.
These lymphoid tissue accumulations cannot be described as lymph
nodes as they do no lie along the pathway of lymphatic ducts.
They do not filter lymph fluid for antigens as lymph nodes do.
These more primitive organs of the immune system still play an
important part in our defense and some have quite unique functions.
Some of these lymphoid accumulations are only temporary. They
appear, migrate and disappear within a few weeks. When challenged
by and infection they become such lymphoid accumulations become
more numerous and persist for longer but once the danger is over
they disappear again. Just behind the epithelial wall of the gut
a sprinkling of lymphocytes is maintained. Sometimes these cells
huddle together and then disperse again. We assume they are involved
in monitoring any breach in the gut wall.
Slightly more permanent and more organized are isolated accumulations
of immune cells that look like a single follicle as seen in lymph
nodes. These follicles are mostly composed of T and B cells. These
follicles are mostly found in the gut wall and closely associated
with the lungs. Again, their position suggests they are involved
with monitoring for pathogens that might penetrate the gut or
respiratory walls. At any one time we have around 25,000 of these
follicles distributed in the gut and lungs.
There are permanent non-encapsulated lymphoid tissues, namely
the tonsils, Peyers patches and appendix. These tissues and the
temporary accumulations described above are collectively called
"mucosa-associated lymphoid tissue". Those lymphoid
tissues exclusively associated with the gut (IE excluding respiratory
associated tissue) are collectively called "gut associated
lymphoid tissue".
Tonsils
We have six separate tonsils, you can only see two of them when
you open your mouth. The two we can see are the palatine tonsils.
Above these, just out of sight at the top of the mouth are the two
tubal tonsils. Above these close to the nasal passage sits a single
pharyngeal tonsil and at the bottom, at the back of the tongue sits
a single lingual tonsil. These tonsils are pretty much the same
in structure. They each sit under the epithelial cell wall and look
shriveled and wrinkled. The wrinkles are called crypts and they
subdivide the interior of a tonsil. Inside the tonsils have multiple
lymphoid follicles again looking like those follicles seen in lymph
nodes. Mostly consisting of B cells plus T cells and APCs the tonsils
respond to antigenic challenge by proliferation of cells and production
of antibody.
Peyers
patches
Along the length of the small intestine are up to 200 oval lumps
of lymphoid tissue. Each is an accumulation of up to 60 lymphoid
follicles composed of B cells with T lymphocytes in the interfollicular
areas.. These patches are situated just beneath the gut epithelium
and gut antigens can readily pass into the lymphoid tissue. The
epithelium covering Peyers patches is modified such that there is
no secretion of mucus and the cells, called "M cells",
readily allow passage of gut antigens straight through them.
Peyers patches are involved in defense against pathogens that
may be colonizing the gut but they are also involved in "oral
tolerance". Food antigens are foreign and strictly speaking
our immune system should be seeing food as an antigenic, potentially
threatening challenge to the body's survival. However, we are clearly
able to absorb food antigens through our gut into our blood stream
with no obvious ill effects. Some of us may be intolerant to certain
food antigens such as lactose in milk or wheat gluten but for the
most part foreign food molecules pass through our bodies unnoticed
by our immune system.
We don't understand the details but it would seem that the Peyers
patches are involved in monitoring food antigens. The Peyers patches
decide what are dangerous gut pathogens to which the body should
respond, and which are food antigens which the body should accept.
The lymphoid tissue is somehow capable of telling the lymphocytes
that they should not respond to food antigens. The lymphocytes are
"tolerized" and become non-responsive to these antigens.
Oral tolerance is a growing area of research. As you may be aware
it is a mechanism that has been suggested could be manipulated to
combat autoimmune disease. The hypothesis is that if we knew the
antigenic target in an autoimmune disease then an individual could
be given purified antigenic material to eat. Presenting the purified
antigen to the gut associated lymphoid tissue could promote tolerance
of the immune system towards this antigen. This mechanism has been
successful in some animal models of autoimmune disease and human
trials of eating purified chicken collagen to reduce rheumatoid
arthritis inflammation has met with some success.
Appendix
So, you were probably told that the appendix is a vestigial organ
that has no function in humans. Well, the body doesn't carry excess
baggage around for nothing. The appendix has been adapted into a
lymphoid organ too. The appendix looks somewhat similar to the tonsils
in that it has crypts penetrating from the gut opening deep into
the tissue and around these are numerous lymphoid follicles.
That concludes our look at organs of the immune system. You can
see that we have a range of organs with different functions. Some
organs are vital to our survival such as the bone marrow. The thymus
is crucial in the development of T cells and the spleen is extremely
important in our defense but we can survive without them with the
aid of modern medicine to keep us healthy. The peripheral organs,
lymph nodes and mucosal associated lymphoid tissue, are expendable
without too much damage to our defense.
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