Lymph node
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Structure of the lymph node.1. Efferent lymphatic vessel 2. Sinus 3. Nodule 4. Capsule 5. Medulla 6. Valve to prevent backflow 7. Afferent lymphatic vessel. |
Lymph nodes are components of the
lymphatic system. They are sometimes incorrectly called "Lymph glands", but they do not secrete anything and are therefore not
glands. Lymph nodes act as filters, with an internal honeycomb of connective tissue filled with
lymphocytes that collect and destroy bacteria and viruses. When the body is fighting an infection, these lymphocytes multiply rapidly and produce a characteristic swelling of the lymph nodes. Humans have approximately 500-600 lymph nodes. Clusters of lymph nodes are found in the underarms, groin, neck, chest, and abdomen.
Lymph nodes are bean-shaped and range in size from a few millimeters to about 1-2 cm in their normal state. They may become enlarged due to a tumor or infection. In some cases, they may feel enlarged due to past infections; although one may be perfectly healthy, one may still feel residually enlarged. The lymph node is surrounded by a fibrous capsule, and inside the lymph node the fibrous capsule extends to form
trabeculae. Thin reticular fibers form a supporting meshwork inside the node. The concave side of the lymph node is called the
hilum. The
artery and
vein attach at the hilum and allow blood to enter and leave the organ, respectively. The
parenchyma of the lymph node is divided into an outer
cortex and an inner
medulla. The cortex is absent at the hilum.
The cortex contains several oval-shaped
lymphoid nodules (also called follicles) which are aggregates of
lymphocytes contained within a meshwork of supporting cells. Nodules that consist mainly of small lymphocytes are called
primary nodules. Nodules called
secondary nodules are those that contain a pale central region called a
germinal center. The germinal center forms (and thus a primary nodule becomes a secondary nodule) when a B cell that has recognized an
antigen undergoes proliferation, differentiates into
plasma cells, and forms
antibodies.
The band of the cortex adjacent to the medulla is the deep cortex, also called the
juxtamedullary cortex or
paracortex. This layer is devoid of nodules. Formation of the deep cortex depends on the migration of
T cells. Thus it is sometimes also called the
thymus-dependent cortex. In comparison, the layer of the cortex that contains nodules is called the nodular cortex.
The medulla consists of cords of lymphatic tissue (
medullary cords) separated by vessel-like spaces called
medullary sinuses.
Lymph travels to the lymph node via
afferent lymphatic vessels and drains into the node just beneath the capsule in a space called the
subcapsular sinus. The subcapsular sinus drains into trabecular sinuses and finally into medullary sinuses. The sinus space is criss-crossed by the
pseudopods of
macrophages which act to trap foreign particles and filter the lymph. The medullary sinuses converge at the hilum and lymph then leaves the lymph node via the
efferent lymphatic vessel.
Lymphocytes, both
B cells and
T cells, constantly circulate through the lymph nodes. They enter the lymph node via the bloodstream and may cross the wall of the bloodvessel by the process of
diapedesis. The B cells migrate to the nodular cortex and medulla, and the T cells migrate to the deep cortex.
When a lymphocyte recognizes an
antigen, B cells become activated and migrate to germinal centers. When antibody-producing plasma cells are formed, they migrate to the medullary cords. Stimulation of the lymphocytes by antigens can accelerate the migration process to about 10 times normal, resulting in characteristic swelling of the lymph nodes.
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The human lymphatic system |
The
spleen and
tonsils are large lymphoid organs that serve similar functions to lymph nodes, though the spleen filters out blood cells rather than bacteria or viruses.
Anterior cervical
These nodes, both superficial and deep, lie above and beneath the
sternocleidomastoid muscles. They drain the internal structures of the
throat as well as part of the posterior
pharynx,
tonsils, and
thyroid gland.
Posterior cervical
These nodes extend in a line posterior to the sternocleidomastoids but in front of the
trapezius, from the level of the
mastoid bone to the
clavicle. They are frequently enlarged during upper
respiratory infections.
Tonsillar
These nodes are located just below the angle of the
mandible. They drain the tonsillar and posterior pharyngeal regions.
Sub-mandibular
These nodes run along the underside of the jaw on either side. They drain the structures in the floor of the mouth.
Sub-mental
These nodes are just below the chin. They drain the teeth and intra-oral cavity.
Supraclavicular
These nodes are in the hollow above the
clavicle, just lateral to where it joins the
sternum. They drain a part of the
thoracic cavity and abdomen.
Virchow's node is a left supraclavicular lymph node which receives the lymph drainage from most of the body (especially the abdomen) via the
thoracic duct and is thus an early site of
metastasis for various malignancies.
These drain the whole of the arm, and are divided into two groups, superficial and deep. The superficial nodes are supplied by lymphatics which are present throughout the arm, but are particularly rich on the palm and flexor aspects of the digits.
Superficial lymph glands of the arm
These comprise the:
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supratrochlear glands, situated above the medial epicondyle of the
humerus, medial to the
basilic vein. They drain the C7 and C8
dermatomes.
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deltoideopectoral glands, situated between the
pectoralis major and
deltoid muscles inferior to the
clavicle.
Deep lymph glands of the arm
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Lymphatics of the axillary region |
These comprise the axillary glands, which are 20-30 individual glands and can be subdivided into:
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lateral glands*
anterior or pectoral glands*
posterior or subscapular glands*
central or intermediate glands*
medial or subclavicular glands*
Adenitis*
Inguinal lymph node*
Lymphadenectomy*
WKU - Immunology Lecture 4, Biology 328 *
Lymph Node
