LYMPHATIC SYSTEM
ly m p h a tic s y ste m
A
system
of vessels (lymphatic vessels)
that drains
l y m p h
from
tissues all over
the
body
back
into
the
bloodstream
. The
lymphatic system
is part of the
i m m u n e
s y s t e m
and
has a
major function
in
defending
the
body
against infection
and
cancer. The
lymphatic system
also
plays a
part in the
absorption
of fats
from
the
intestine.
STRUCTURE AND FUNCTION
A
ll body
tissues are
bathed
in lymph, a
watery
fluid that is derived
from
the
bloodstream
. Much
of this fluid is
returned
to
the
bloodstream
through
the
walls of the
capillaries (see
c i r c u l a -
t o r y s y s t e m
), but the
remainder of the
fluid is transported
to
the
heart through
the
lymphatic system
.
Lymph
is moved
along
the
lymphatic
vessels
during
physical
activity,
in
which
the
contraction
of muscles com
-
presses the
vessels; valves inside the
vessels ensure
that the
lymph
always
flows in the
correct direction.
S
ituated
on
the
lymphatic vessels are
l y m p h n o d e s
,
through
which
the
lymph
L
passes. These
nodes filter the
lymph
and
trap
or destroy
infectious m
icroorg-
anisms or other foreign
bodies. A
lso
included
in the
lymphatic system
are
the
s p l e e n
and
the
t h y m u s
,
which
pro-
duce
l y m p h o c y t e s
(white blood
cells that
fight infection
or harm
ful cells).
DISORDERS
If infection
or inflammation
occurs in
any
part of the
body, the
lymph
nodes
in that particular area
may
become
swollen
and
tender as the
white blood
cells w
ithin
them
try
to
combat the
organisms or cells causing
the
problem
.
If an
infection
is particularly severe, the
lymphatic vessels
leading
from
the
nodes may
also
become
inflamed
(see
l y m p h a n g i t i s
).
If a
lymphatic vessel becomes ob-
structed
(for example, by
worms or
by
cell debris), lymph
w
ill collect in
the
nearby
tissues, which
w
ill lead
to
l y m p h o e d e m a
.
Cancer commonly
spreads to
other
parts of the
body
through
the
lymph-
atic system
. If a
primary tumour invades
the
lymphatic vessels, cells from
the
tu-
mour may
break
off and
enter the
vessels (see
m e t a s t a s i s
). They
may
then
become
lodged
in lymph
nodes situated
nearby, where
they
may
then
grow
into
secondary
tumours. In
b r e a s t c a n c e r
,
for example, the
cells from
a
breast
tumour may
spread
to
the
lymph
nodes
in the
armpit.
ly m p h g la n d
A
popular name
for a
l y m p h n o d e
.
(See
also
l y m p h a t i c s y s t e m .
)
ly m p h n o d e
A
small organ
lying
along
the
course
of a
lymphatic vessel (see
l y m p h a t i c
s y s t e m
). Lymph
nodes are
commonly,
but incorrectly, called
lymph
glands.
Lymph
nodes vary
in size; they
can
be
m
icroscopic or can
be
up
to
about
2.5 cm
in diameter.
STRUCTURE
A
lymph
node
is composed
of a
thin,
fibrous outer capsule
and
an
inner mass
of lymphoid
tissue. Penetrating
the
cap-
sule
are
several small lymphatic vessels,
which
transport lymph
into
the
node.
Each
lymph
node
contains
sinuses
(spaces) in which
the
lymph
is filtered.
A
single, larger vessel carries lymph
out
of the
node.
FUNCTION
Lymph
nodes act as a
barrier to
the
spread
of infection
by
destroying
or fil-
tering
out bacteria
before
they
can
pass
into
the
bloodstream
. As lymph
passes
through
a
node, narrow
channels in
the
sinuses slow
down
its movement;
this reduction
in the
flow
of lymph
allows macrophages (white blood
cells
that engulf and
destroy
foreign
and
dead
material) to
filter m
icroorganisms
from
the
lymph. In addition, germ
inal
centres, which
are
located
in the
lymph
node, release
white blood
cells known
as
l y m p h o c y t e s
.
These
cells also
help
to
fight infection.
ly m p h o c y te
Any
one
of a
group
of white
b l o o d c e l l s
that are
of crucial importance
to
the
i m m u n e
s y s t e m
because
they
combat
infectious organisms and
cancer. There
are
two
principal types of lymphocyte:
B- and
T-lymphocytes.
B-LYMPHOCYTES
B-lymphocytes produce
i m m u n o g l o b u -
l i n s
or
a n t i b o d i e s
(proteins that attach
themselves to
a n t i g e n s
(proteins) on
the
surfaces of bacteria). This action
starts a
process that leads to
the
destruction
of
the
bacteria.
T-LYMPHOCYTES
The
T-lymphocytes are
classified
in two
main
groups, according
to
the
type
of
antigens found
on
their surfaces. One
group
(known
as CD8) includes killer
(cytotoxic) and
suppressor cells; the
other group
(CD4)
includes helper
cells. The
killer T-lymphocytes attach
themselves to
abnormal cells
(such
as tumour cells, cells that have
been
invaded
by
viruses, and
those
in trans-
planted
tissue) and
release
chem
icals
that help
to
destroy
the
abnormal cells.
Suppressor T-cells act to
“damp
down”
the
immune
response. Helper T-cells
enhance
the
activities of the
killer
T-cells and
the
B-cells, and
also
orches-
trate
the
immune
response.
MEMORY FUNCTION
Some
B- and
T-lymphocytes do
not par-
ticipate
directly in immune
responses
but serve
as a
memory
bank
for anti-
gens that have
been
encountered. These
lymphocytes
are
created
when
the
immune
system
cells respond
to
an
infection, and
they
contain
information
on
how
to
deal w
ith
that infection.
Once
the
immune
response
is over, the
memory
cells are
then
stored
in the
body
in
order
to
aid
subsequent
immune
responses to
that infection.
ly m p h o c y to s is
An
increase
in the
number of
l y m p h o -
c y t e s
,
a
type
of white blood
cell that
mounts the
immune
response, either by
attacking
foreign
cells directly or by
producing
antibodies that cause
their
destruction. Lymphocytosis, which
can
be
detected
by
blood
test, most com
-
monly occurs in viral infections. Rarely,
a
massive
increase
in lymphocytes may
be
due
to
l e u k a e m i a
.
ly m p h o e d e m a
An
abnormal accumulation
of
l y m p h
in
the
tissues, which
occurs when
the
nor-
mal drainage
of lymph
is disrupted
(see
l y m p h a t i c s y s t e m
).
CAUSES
There
are
various possible
causes for
lymphoedema. Cancer can
lead
to
the
condition
if the
vessels become
blocked
by
deposits of cancer cells. Surgical re-
moval of
l y m p h n o d e s
under the
arm
(as
in radical
m a s t e c t o m y
) or in the
groin,
or
r a d i o t h e r a p y
to
an
area
containing
lymph
nodes, may
also
result in lymph-
oedema. In the
tropical disease
f i l a r i a s i s
,
the
lymphatic vessels may
be
blocked
by
parasitic worms. Rarely, the
condition
is
due
to
a
c o n g e n i t a l
abnormality
of the
lymphatic vessels known
as M
ilroy’s
d
isease. In addition, lymphoedema
may
occur for no
known
reason.
SYMPTOMS
Lymphoedema
causes painless swelling
in the
tissues and
thickening
of the
skin
over the
affected
area. It usually devel-
ops in the
legs; the
swelling
may
start at
the
ankle
and
extend
up
the
leg, and
480
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