• Clinical science

Lymphatic system

Primary lymphatic organs

The primary lymphatic organs are the bone marrow and thymus. They are the site at which lymphocytes form and mature. 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


The Thymus arises from the Third pharyngeal pouch.

DiGeorge syndrome is caused by an 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 viral as well as fungal infections, and hypocalcemia.

Secondary lymphatic organs

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.


Lymph nodes

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

  • Function: immune system activation in response to ingested pathogens
    • M cells transport antigens from the intestinal lumen to antigen-presenting cells (e.g., macrophages) → B cells within the germinal center detect antigens and differentiate to plasma cells that secrete IgAIgA acquires a secretory component and becomes secretory IgA → secretory IgA is transported across the intestinal epithelium and secreted on the luminal surface of the gut
  • Location: ileum (lamina propria and submucosa)
  • Structure
    • Similar to other secondary lymphatic organs
    • No surrounding capsule
  • Histology
    • Aggregates of lymphoid follicles: contain numerous lymphocytes and dendritic cells
    • Follicle-associated epithelium : site of specialized M cells


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

Palatine tonsils

Lingual tonsils Tubal tonsils
  • Roof and posterior wall of the nasopharynx
  • Tonsillar fossa on both sides of the oropharynx
  • Base of the tongue
  • Lateral wall of the nasopharynx

Lymphatic drainage

The thoracic duct is a lymphatic structure that originates in the abdomen and travels through the thorax to drain in the venous system. It carries both emulsified fat and lymph, which together are referred to as chyle. Injury to the lymphatic channels, particularly the thoracic duct, causes chylothorax.

Lymph node clusters

Palpable lymph nodes

Head and neck

Cluster Location Drainage area Differential diagnoses
Parotid lymph nodes
  • Skin of the ear, cheek, and forehead
  • 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
  • Tongue, gum, cheek, lips

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
  • Localized infections of the scalp (e.g., lice, fungal), rubella, measles

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

Cluster Location Drainage area Differential diagnoses
Axillary lymph nodes Central lymph nodes
  • Lie in axillary fat

Anterior (pectoral) lymph nodes

  • Anterior axillary fold
  • Mammary and pectoral regions

Posterior (subscapular) lymph nodes

  • Posterior axillary fold
  • Upper back and posterior neck
  • Localized infection of the upper extremities/chest wall

Lateral (brachial) lymph nodes

  • Medial, proximal upper arm
  • Majority of the upper limb
  • Localized infections of the upper extremities

Apical (subclavicular) lymph nodes

  • Infraclavicular
Supratrochlear/epitrochlear lymph nodes

Lower extremity

Cluster Location Drainage area Differential diagnoses
Inguinal lymph nodes Superficial inguinal lymph nodes
  • Superficial fascia of the thigh in the region of the femoral triangle
  • Arranged in the shape of “T”
    • The upper horizontal group lie along the inguinal ligament
    • The lower vertical group lie along the terminal part of the long saphenous vein
Deep inguinal lymph nodes

Popliteal lymph nodes

  • Popliteal fossa

The testicles, epididymis, and seminal ducts are part of the internal genital organs and are drained by the deep, iliac, and lumbar lymph nodes.

Nonpalpable lymph nodes



Location Drainage area Differential diagnoses
Mediastinal lymph nodes (paratracheal)
Hilar lymph nodes
  • Adjacent to main stem bronchus (hila of the lungs)


Cluster Location Drainage area Differential diagnoses
Pre-aortic lymph nodes Celiac lymph nodes
Superior mesenteric lymph nodes
Inferior mesenteric lymph nodes
  • Colon from splenic flexure to the uppermost part of the rectum → cisterna chyli
Para-aortic lymph nodes (lumbar)


Cluster Location Drainage area Differential diagnoses
Internal iliac lymph nodes
  • Surrounding the internal iliac arteries
External iliac lymph nodes
  • Around the external iliac arteries
Common iliac lymph nodes
  • Around the common iliac vessels


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  • 2. Lucey BC, Stuhlfaut JW, Soto JA. Mesenteric lymph nodes seen at imaging: causes and significance. Radiographics. 2005; 25(2): pp. 351–365. doi: 10.1148/rg.252045108.
  • Kaplan. USMLE Step 1 Anatomy Lecture Notes 2016. Kaplan Publishing; 2015.
  • Murphy K, Weaver C. Janeway's Immunobiology. Garland Science; 2016.
  • Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
last updated 09/01/2019
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