Introduction to lymph nodes

Cortex

Within the cortex are regions of lymphocyte aggregation (primarily B-cells and some supporting T-cells) and specialized follicular dendritic cells that form the lymphoid follicles or lymphatic nodules. These can either be primary or secondary follicles depending on their cellular population. Primary lymphoid follicles contain small, dormant lymphocytes, while secondary lymphoid follicles contain a lighter staining area of active lymphocyte proliferation known as a germinal centre. The germinal centre supports affinity maturation (high affinity antibody production) of B-cells. It is subdivided into a dark zone, light zone and a mantle zone. The B-cells of the dark zone are known as centroblasts. They rapidly replicate, resulting in hypermutation of their antibody molecules. Centroblasts migrate to the light zone, where they are referred to as centrocytes. Here they compete for binding with the unprocessed antigens presented on the surface of follicular dendritic cells. Those centrocytes that successfully bind to the follicular dendritic cells will survive, while the others will die. Small, quiescent cells are peripherally marginalized due to the rapid proliferation of the central cells. These cells form the mantle zone of the germinal centre.

Paracortex

Deep to the cortical layer and superficial to the medulla is the paracortex. This region contains mostly T-cells of the CD4 (cluster of differentiation) and CD8 subsets. Migrating dendritic cell lines (such as Langerhans cells) found in this area present processed antigen to the T-cells.

Medulla

The lymphocytes of the medulla are less organized and form irregular medullary cords. The cords also contain plasma cells, small lymphocytes and macrophages. The medullary sinuses drain the lymph coming from the cortical sinuses to the efferent lymphatic vessel via the hilum.

Test your knowledge on the histology of the lymph node with this quiz.

Brain

It is a long held concept that the brain is the only region of the body devoid of a lymphatic system. The idea was that resident microglia were totally responsible for maintaining immunity within the region. However, the method by which these macrophages entered the central nervous system was still unclear. It has been recently proposed by Louveau et al. (2015) that lymphatic channels line the dural sinuses located between the two layers of dura and drain their contents to the deep group of cervical lymph nodes.

Cervical

The cervical lymph nodes can be subdivided into two major groups. Those superficial to the sternocleidomastoid muscle are known as the superficial cervical nodes, while those deep to the same muscle are the deep cervical nodes. The superficial cervical nodes are further subdivided into the pre-auricular or parotid nodes (anterior to the external ear), mastoid nodes (posterior to the external ear), and the occipital nodes. The deep cervical nodes are located in relation to the internal jugular vein. The superior deep cervical nodes are adjacent to the upper part of the internal jugular vein, while the inferior deep cervical nodes are adjacent to the lower part of the same vein.

Waldeyer's ring

The nasal and oral passages are two of the major ports of entry that pathogens use to access the human body. The region is guarded by a collection of lymphatic tissue known as Waldeyer’s ring. This circular collection of lymphoid tissue is formed by the pharyngeal tonsils (adenoids) located in the fossa of Rosenmüller, the tubal tonsils of Gerlach in the torus tubaris, the palatine tonsils between the palatoglossal and palatopharyngeal folds, the lingual tonsils at the posterior region of the tongue and mucosa-associated lymphatic tissue lining the oropharyngeal wall.

Axillary

The axillary lymph nodes were previously divided into three groups by pectoralis minor. Level one nodes are inferior to the muscle, level two nodes are posterior to the muscle and level three nodes are superior to the muscle. Subsequently, axillary nodes have been divided into five groups: 

• apical
• central (in the fat of the axilla)
• posterior (subscapular)
• anterior (pectoral)
• lateral (medial to the axillary vein) nodes.

Epitrochlear

Epitrochlear or supratrochlear nodes are located superficial to the deep fascia of the arm and medial to the basilic vein and proximal to the medial epicondyle.

Mediastinal

Mediastinal lymph nodes are divided into nine stations. They include those found in the hilum of the lungs, the juxta-oesophageal nodes, and superior and inferior tracheobronchial nodes. There are also posterior mediastinal nodes. The thymus, which is a primary lymphatic organ, is also found in the anterior mediastinum.

Abdominal

Throughout the abdominopelvic region there are numerous groups of nodes associated with specific viscera and adjacent to the main vascular structures within the region. Those associated with the vascular structures include the para-aortic and mesenteric (superior and inferior) nodes, common, internal and external iliac nodes, and the superior and middle rectal nodes.

Learn everything about the lymphatic drainage of the abdomen and pelvis.

Inguinal

The inguinal lymph nodes are defined as superficial or deep based on their relationship to the fascia lata of the thigh. The group of nodes superficial to the fascia lata are the superficial inguinal nodes. They are further subdivided into inferior, superolateral and superomedial nodes. Those nodes deep to the fascia are the deep inguinal nodes. The largest and most superior of the deep nodes is the deep inguinal node of Cloquet. It is located in the femoral ring.