the bloodstream through the walls of the
capillaries (see
circulatory system
the remainder is transported to the heart
through the lymphatic system.
Lymph is moved along the lymphatic
vessels during physical activity, as mus-
cle contractions compress the vessels;
valves inside the vessels ensure that the
lymph flows in the correct direction. Sit-
uated on the lymphatic vessels are
through which the lymph passes.
These nodes filter the lymph and trap
infectious microorganisms or other for-
eign bodies. The nodes contain many
, white blood cells that can
neutralize or destroy invading bacteria
and viruses. The lymphatic system also
includes the
and the
which produce lymphocytes.
lymph gland
A popular name for a
lymph node
(See also
lymphatic system.)
lymph node
A small organ lying along
the course of a lymphatic vessel (see
lymphatic system
); commonly but incor-
rectly called a lymph gland. Lymph nodes
vary considerably in size, from micro-
scopic to about 2.5 cm (1 in) in diameter.
A lymph node consists of a thin, fibrous
outer capsule and an inner mass of lym-
phoid tissue. Penetrating the capsule are
several small lymphatic vessels (which
carry lymph into the node). Each node
contains sinuses (spaces), in which the
lymph is filtered. The flow of the lymph
slows as it moves through narrow chan-
nels in the sinuses; this reduction in flow
allows macrophages (white blood cells
that engulf and destroy foreign and dead
material) time to filter microorganisms
from the lymph. Germinal centres in the
lymph node release white blood cells
, which also help to
fight infection. A single, larger vessel
carries lymph out of the node.
Any one of a group of white
blood cells
that are of crucial impor-
tance to the
immune system
. There are
principal types of lymphocyte: B- and
T-lymphocytes. B-lymphocytes produce
attach themselves to
on the surfaces of bacteria. This starts a
process leading to the destruction of the
bacteria. The T-lymphocytes comprise 3
main groups of cells: killer (cytotoxic)
cells, helper cells, and suppressor cells.
The killer T-lymphocytes attach to abnor-
mal cells (for example, tumour cells,
cells that have been invaded by viruses,
and those in transplanted tissue) and
release chemicals called lymphokines,
which help to destroy the abnormal
cells. Helper T-cells enhance the activi-
ties of the killer T-cells and the B-cells,
and also control other aspects of the
immune response. Suppressor T-cells act
to "switch off" the immune response.
Some lymphocytes do not participate
directly in immune responses, but serve
as a memory bank for antigens that
have been encountered,
lym phoedem a An abnormal accumula-
of lymph
in the tissues, which occurs
when the normal drainage of lymph is
disrupted (see
lymphatic system)
are various causes. In the tropical dis-
lymphatic vessels may be
blocked by parasitic worms. Cancer can
lead to lymphoedema if vessels become
blocked by deposits of cancer cells. Sur-
gical removal of
Ijmnph nodes
under the
arm or in the groin, or
an area containing lymph nodes, may
also result in lymphoedema. Rarely, the
condition is due to a
mality of the lymphatic vessels known
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