• Secondary lymphoid tissue has many important roles in immunity including
    • filtration and sampling of bodily fluids in order to detect infection
    • sequestration of pathogens and ingestion by innate immune cells
    • activation of adaptive immunity by antigen presenting cells
    • maintainance of immunological memory by storage of long lived cells
  • There are several types of secondary lymphoid tissue including the
    • spleen
      • a solid organ in the left upper quadrant of the abdomen
    • lymph nodes
      • a diffuse network of small nodes throughout the body
    • mucosal lymphoid tissue
      • patches of immunological tissue such as
        • Waldeyer's ring consisting of
          • tubular tonsils in the pharyngeal recess
          • palatine tonsils
          • lingual tonsils at the base of the tongue
        • Peyer patches in the lower jejunum and ileum

  • The spleen is located in the left upper quadrant of the abdomen where
    • it is protected by the 9th to 11th ribs
    • it sits anterior to the left kidney
  • There are several components of the spleen, which are divided into
    • white pulp, which is the
      • the immunologically active part of the spleen containing
        • germinal centers containing
          • activated B-cells (part of a larger B-cell follicle)
        • periarteriolar lymphatic sheaths (PALS) containing
          • T-cells
        • marginal zones containing
          • macrophages and other antigen-presenting cells
    • red pulp, which is
      • the hematologically active part of the spleen characterized by
        • arterioles to allow for
          • delivery of red blood cells (RBCs)
        • resident macrophages to ingest both
          • dying RBCs and encapsulated bacteria
        • sinusoids with a fenestrated basement membrane to
          • allow for red RBC exit
  • There are many clinical manifestations of asplenia including
    • increased susceptibility to infections by encapsulated bacteria
    • appearance of abnormal RBC forms on peripheral blood smear
    • others described more fully in the asplenia topic
Lymph Nodes

  • Lymph nodes are a diffuse network of encapsulated lymphoid tissue that
    • have many regional afferent lymphatic vessels
    • have at least one efferent lymphatic
  • There are several components of a lymph node, which are divided intothe
    • cortex
      • an outermost layer of the lymph node that is composed of
        • follicles composed of proliferating B cells that can be
          • dormant and cell dense primary follicles
          • active and pale secondary follicles
        • subcapsular macrophages that monitor incoming lymph
        • dendritic cells that serve as antigen presenting cells
    • paracortex
      • an intermediate layer of the lymph node that is composed of
        • high endothelial vessels (HEVs) where 
          • lymphocytes enter lymph nodes
        • T-cells that await activation, meaning that this region is
          • hypertrophied in infections triggering cellular immunity
          • underdeveloped in patients with DiGeorge syndrome
    • medulla
      • an innermost layer of the lymph node that is composed of
        • cords with closely packed lymphocyte and plasma cell "cords"
        • sinuses that drain into the efferent lymph vessels and house
          • reticular cells
          • macrophages
Lymph Node Drainage Pattern
Lymph Node Drainage Pattern
Lymph Node Cluster
Drainage Area


  • Head
  • Neck


  • Lungs
  • Trachea
  • Esophagus
  • Upper limb
  • Breast
  • Skin above the umbilicus
  • Liver
  • Stomach
  • Spleen
  • Pancreas
  • Upper duodenum
Superior mesenteric
  • Lower duodenum
  • Jejunum
  • Ileum
  • Colon proximal to splenic flexure
Inferior mesenteric
  • Colon between splenic flexure and upper rectum
Internal iliac
  • Lower rectum
  • Anal canal proximal to pectinate line
  • Bladder
  • Cervix
  • Prostate
  • Testes
  • Ovary
  • Kidney
  • Uterus
Superficial inguinal
  • Anal canal distal to pectinate line
  • Skin below the umbilicus
  • Scrotum
  • Vulva
  • Dorsolateral foot
  • Posterior calf



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