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Blueboom/Immune and Lymphoid Tissue

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The Lymphoid System


  • concentrates antigen in lymphoid organs
  • circulates lymphocytes in organs to contact
  • conveys products of the immune system to other tissues /organs
  • eliminates antigens and worn out self components

Primary vs. Secondary

Primary vs. Secondary Primary Lymphoid Organs include:

  • bone marrow
  • thymus

Secondary Lymphoid Organs include:

  • lymph nodes
  • spleen
  • tonsils

B cells proliferate in germinal centers and then differentiate into plasma cells and memory cells.


  • has a Connective Tissue capsule and is lobulated
  • contains Hassall's corpuscules (found in the medulla) which are rings of degenerating epithelial reticular cells (ERC)
  • Mature T cells found in the medulla
  • No lymph nodes
  • Efferent lymph vessels only
  • No germinal centers
  • Primary Lymphoid Organ
  • Generates competent T lymphocytes

Cortex: contains lymphocytes and epithelial reticular cells Medulla: contains less concentrated lymphocytes

Blood-Thymus barrier

Has 5 components

  • Capillary endothelial cells
  • Capillary basal lamina
  • Perivascular connective tissue
  • Epithelial reticular basal lamina
  • Epithelial reticular cells

Lymph nodes

  • has a CT capsule (Dense Irregular Connective Tissue + some elastic fibers + Smooth Muscle)

**usually surrounded by adipose tissue

  • has afferent and efferent vessels
  • Cortex : lymph nodes and diffuse lymph tissue
  • Medulla: light and central

Flow of lymph:

Afferent lymphatic vessels > capsule > subcapsular sinuses > intermediate Sinuses > medullary sinuses > hilus > efferent lymphatic vessel

3 Types of Sinuses

  1. Subscapular sinus
  2. Trabecular sinus between lymph tissue
  3. Medulary sinus - related to efferent lymph vessel

High endothelial venules (HEV)

- are in the deep cortex/medulla - allow for transition of lymphocytes from the blood stream to lymph tissue - look for the squeezing of the lymphocytes


  • Simple squamous epithelium
  • has a CT capsule which extends interior as trabecule
  • Contains nodules throughout organ
  • No cortex/medulla
  • Central arteries sometimes present

White Pulp

  • lymph nodules + PALS
  • PALS contain the largest amount of T lymphocytes

Red Pulp

Contains Macrophages + Reticular fibers

Sometimes the trabeculum is surrounded by lymphocytes (Periarterial Lymphatic sheath (PALS)

Venous sinuses

*open circulation blood vessel terminates in red pulp *closed circulation blood vessels are continuous


  • Has a CT Capsule
  • Contains No cortex / medulla
  • Lots of nodules

Tonsil Types

  • Palatine tonsils have stratified squamous epithelium
  • Pharyngeal tonsil have cilliated pseudostratified epithelium
  • Lingual tonsil contains mucous glands

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This page was last modified on 6 February 2008, at 08:04.
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