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Lymphatic system

Last updated: June 9, 2021

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The lymphatic system is part of the adaptive immune system as well as the circulatory system and comprises the thymus and bone marrow (primary lymph organs); mucosa-associated lymphatic tissue (MALT), the spleen, and the lymph nodes (secondary lymphatic organs); the lymphatic vessels and capillaries; and the lymph fluid. The primary function of the lymphatic system is to return excess interstitial fluid and waste products, such as proteins and cellular debris, to the bloodstream. The interstitial fluid is absorbed by lymphatic capillaries throughout the body via diffusion. The lymph fluid is then transported through the network of lymphatic vessels and lymph nodes to the right lymphatic duct, which drains into the right subclavian vein, and the thoracic duct, which drains into the left subclavian vein. Its secondary function is immune defense, which mainly involves the transport of leukocytes (esp. lymphocytes) between the bone marrow and the lymph nodes and the stimulation of immune response through the transport of antigen-presenting cells to the lymph nodes. Another function is the transport of fats as chyle from the digestive system to the bloodstream. Accordingly, lymph fluid is transparent when initially formed from the interstitial fluid but adopts a milky appearance as it accumulates proteins, fats, cellular debris, and leukocytes.
The lymph nodes are the main sites for lymph filtration and the storage of lymphocytes, including B cells, which mature and differentiate in the bone marrow, and T cells, which likewise form in the bone marrow but migrate to the thymus for maturation. Antigen presentation induces the differentiation and proliferation of B lymphocytes and the activation of T lymphocytes. Following antigen presentation, mature lymphocytes differentiate into effector cells in the secondary lymph organs. Lymph node clusters exist throughout the body, some of which are palpable (e.g., cervical lymph nodes) while others are not (e.g., mediastinal lymph nodes). They may become enlarged in response to inflammatory processes due to local infection, malignancy, or granulomatous disease.

Lymphocytes form and mature in the primary lymphatic organs (the bone marrow and thymus). Both B lymphocytes and T lymphocytes arise from hematopoietic stem cells in the bone marrow. While B lymphocytes remain within the bone marrow during the process of maturation, T lymphocytes migrate to the thymus to mature and differentiate.

Bone marrow

Bone marrow: B cell maturation; Thymus: T cell maturation

Thymus

The Thymus arises from the Third pharyngeal pouch.

DiGeorge syndrome is caused by abnormal development of the 3rd and 4th pharyngeal pouches, which prevents the formation of thymus and parathyroid glands. As a result, patients with DiGeorge syndrome have an increased susceptibility to hypocalcemia and viral and fungal infections.

The secondary lymphatic organs are the spleen, lymph nodes, and mucosa-associated lymphatic tissue (e.g., the Peyer patches and tonsils). It is in the secondary lymphatic organs that antigen presentation occurs. They are also the site of differentiation of mature, naive lymphocytes into effector cells.

Spleen

Lymph nodes

Function

Location

  • Throughout the body in close proximity to organs and large vessels
  • See “Lymph nodes clusters” below.

Structure

  • Bean-shaped organ surrounded by fibrous capsule with trabeculae
  • The trabecular sinus leads lymph from the subcapsular sinus to the medullary sinus.
  • The hilus
    • Allows blood vessels and efferent lymphatic vessels to enter or leave the lymph node
    • Typically, only one or a small number of efferent lymphatic vessels leave the lymph node, compared to the larger number of afferent lymphatic vessels that enter the lymph node through the cortex.
  • The entering artery and vein branch into a large capillary network, which forms the post-capillary high endothelial venules.

Histology

Clinical significance

Mucosa-associated lymphoid tissues (MALT)

MALT include the tonsils, Peyer patches, and solitary lymphoid follicles of the mucosa. The structure of MALT resembles that of other secondary lymphatic organs but it is also composed of a specialized reticular epithelium (follicle-associated epithelium) with a humoral defense mechanism . The follicle-associated epithelium of the gut-associated lymphatic tissue (GALT) contains M cells, which allow transcytosis of antigens into the lamina propria.

Peyer patches

Tonsils

  • Function: Waldeyer tonsillar ring (first line of defense against inhaled or ingested pathogens)
  • Structure: similar to other secondary lymphatic organs
  • Location and histology
Characteristics of tonsils
Characteristic Pharyngeal tonsils (adenoids)

Palatine tonsils

Lingual tonsils Tubal tonsils
Location
Histology

Overview

Lymph node clusters

Palpable lymph nodes

Head and neck

Lymph nodes of the head and neck
Cluster Location Drainage area Differential diagnoses
Parotid lymph nodes
  • Bacterial, viral, or fungal infections
Preauricular lymph nodes
  • In front of the tragus
Retroauricular lymph nodes

Submandibular lymph nodes

  • Area surrounding the mandibular angle, and between the chin and mandibular angle

Submental lymph nodes

  • Below the chin
  • Base of the mouth, tongue, lower lip
  • Tumors of the oral cavity, localized infections in the drainage area

Occipital lymph nodes

  • Area surrounding the neck and back of the head
  • Back of the head, neck

Deep cervical lymph nodes

Posterior triangle lymph nodes

  • Localized infections in the drainage area

Supraclavicular lymph nodes

  • Supraclavicular fossa, closer to the sternal end of the clavicle

Upper extremity

Lymph nodes of the upper extremity
Cluster Location Drainage area Differential diagnoses
Axillary lymph nodes Central lymph nodes
  • Lie in axillary fat

Anterior (pectoral) lymph nodes

  • Mammary and pectoral regions
  • Skin region located above the umbilicus

Posterior (subscapular) lymph nodes

  • Localized infection of the upper extremities/chest wall

Lateral (brachial) lymph nodes

  • Majority of the upper limb
  • Localized infections of the upper extremities

Apical (subclavicular) lymph nodes

  • Infraclavicular
Supratrochlear/epitrochlear lymph nodes

Abdomen and lower extremity

Lymph nodes of the lower extremity
Cluster Location Drainage area Differential diagnoses
Periumbilical [3]
Inguinal lymph nodes Superficial inguinal lymph nodes
Deep inguinal lymph nodes

Popliteal lymph nodes

The testicles, epididymis, and seminal ducts are drained by the deep, iliac, and lumbar lymph nodes.

Nonpalpable lymph nodes [4][5]

Thoracic

Thoracic lymph nodes

Cluster

Location Drainage area Drain into Differential diagnoses
Mediastinal lymph nodes (paratracheal)
Hilar lymph nodes

Abdominal

Abdominal lymph nodes
Cluster Location Drainage area Drain into Differential diagnoses
Pre-aortic lymph nodes Celiac lymph nodes
Superior mesenteric lymph nodes
Inferior mesenteric lymph nodes
Paraaortic lymph nodes (lumbar)

Pelvic

Pelvic lymph nodes
Cluster Location Drainage area Drain into Differential diagnoses
Internal iliac lymph nodes
External iliac lymph nodes
Common iliac lymph nodes
  • Around the common iliac vessels

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