Immune histology

• T lymphocytes (T cells)


  Thymus dependent

  
• B lymphocytes (B cells)

  Bone marrow or bursa dependent
• Stromal cells


  Form a supportive framework for lymphocytes

  
• Primary lymphatic organs


  • Produce T and B lymphocytes
• Secondary lymphatic organs


  • Organized for surveillance, immune response and immunologic memory

  
• Primary lymphatic organs


  • Bone marrow
  • Thymus
  • Cloacal bursa (birds)
• Secondary lymphatic organs


  • Mucosa associated lymphatic tissue (MALT)
  • Lymph nodes
  • Spleen
  • Hemal nodes
• Thymus


  • Lobed organ located in the neck and cranial mediastinum
  • Thymus is fully formed and functional at birth
  • At puberty the thymus begins to degenerate

  
• Thymus


  • Lobulated organ
  • Each lobule has an outer cortex and an inner medulla
  • Cortex stains dark because of the presence of small, closely packed T lymphocytes

  
• Thymus cortex


  • Epithelial reticular cells
  • Lymphocytes
  • Tingible body macrophages

  
• Epithelial reticular cells


  • Have pale, round nuclei
  • Long, branched cytoplasmic processes
  • Cytoplasmic processes of adjacent cells are connected by desmosomes
  • Cytoplasm contains intermediate filaments
  • Form a three-dimensional network (reticulum) which supports developing lymphocytes

  
• Tingible body macrophages


  • Located close to the medulla
  • Phagocytose dead T lymphocytes
  • Cytoplasm contains material in various stages of degeneration (tingible bodies)

  
• Thymus medulla


  • Contains epithelial reticular cells and loosely packed T lymphocytes
  • Medulla stains less intensely than the cortex because of the presence of large lymphocytes with pale staining nuclei
  • Medulla contains thymic (Hassall's) corpuscles

  
• Thymic/Hassall's corpuscles

  • Composed of closely-packed, concentrically arranged epithelial reticular cells
  • A large keratinized cell forms the centre of the corpuscle
  • Secretory products (thymosin and thymopoietin) of the corpuscles are involved in T lymphocyte differentiation

  
• Thymic/Hassall's corpuscles (ultrastructure)


  • Epithelial reticular cells forming the corpuscles contain keratohyalin granules, intermediate filaments and lipids droplets
  • Cells are connected by desmosomes

  
• Building blocks of secondary lymphatic organs

  • Diffuse lymphatic tissue
  • Lymphatic nodules (follicles)
• Diffuse lymphatic tissue


  • Contains lymphocytes, lymphoblasts and macrophages
  • Occurs in connective tissue of the gut, respiratory tract, urogenital system, skin and between lymphatic nodules

  
• Lymphatic nodules


  • Composed of concentrations of lymphocytes, follicular dendritic cells and macrophages

  
• Follicular dendritic cells


  • Usually found in the germinal centres of nodules
  • Have long, thin cytoplasmic processes
  • Antigen-antibody complexes bind to receptors on the cytoplasmic processes
  • B lymphocytes are located between the cytoplasmic processes

  
• Primary lymphatic nodules


  • Contain connective tissue, immature follicular dendritic cells, lymphocytes (mainly naïve B lymphocytes)
  • Lack a germinal centre
• Secondary lymphatic nodules


  • Contain a germinal centre (light-staining)
  • Mantle zone (dark-staining)

  
• Germinal centre


  • Contains B lymphocytes with light-staining nuclei, follicular dendritic cells, macrophages

  
• Mantle zone


  • Contains small B lymphocytes with dark-staining nuclei

  
• Mucosa-associated lymphatic tissue (MALT)


  • Diffuse lymphatic tissue and/or lymphatic nodules in mucosa of pharynx, mammary gland, respiratory system, alimentary tract, urogenital tract
• MALT


  • Tonsils
  • Gut-associated lymphatic tissue (GALT)
• Tonsils


  • Contain accumulations of lymphatic nodules
  • Pharyngeal tonsils ("adenoids") located in the roof of the pharynx
  • Palatine tonsils located in the lateral walls of the pharynx
  • Lingual tonsils located at the base of the tongue

  
• Palatine tonsil in dog and cat

  • Located in a tonsillar fossa
  • Covered by a fold of mucosa (semilunar fold)
  • Surface is smooth
• Palatine tonsil in horse and ruminants

  • Surface has invaginations (tonsillar fossulae) which extend into tonsillar crypts
  • Walls of tonsillar crypts contain numerous lymphatic nodules
  • Lymphatic nodules usually have a germinal centre and mantle layer
  • Diffuse lymphatic tissue occurs between the lymphatic nodules

  
• Palatine tonsil in all species

  • Overlying epithelium is stratified squamous
  • Epithelium is infiltrated with lymphocytes, neutrophils, macrophages
• Tonsils do not have afferent lymphatic vessels
• Lymphatic capillaries drain lymph from tonsils into efferent lymphatic vessels
• Gut-associated lymphatic tissue (GALT)


  • Solitary and aggregated lymphatic nodules
  • Intraepithelial lymphocytes
  • Sub-epithelial lymphocytes
  • Plasma cells
  • Macrophages

  
• Antigen recognition and mucosal immune responses in the intestines occur in GALT

  
• GALT in the small intestine are also known as Peyer's Patches
• Composition of Peyer's patches

  • Germinal centres of submucosal lymphatic nodules
  • Mantle zones of submucosal lymphatic nodules
  • Internodular region
  • "Domes" above the lymphatic nodules
  • Nodule-associated epithelium

  
• Germinal centres of Peyer's patch lymphatic nodules

  • Contain proliferating B lymphocytes, follicular dendritic cells, macrophages

  
• Mantle zones of Peyer's patch lymphatic nodules


  • Cap the lymphatic nodules and contain small lymphocytes

  
• Internodular region of Peyer's patches

  • Formed by diffuse lymphatic tissue between the lymphatic nodules
  • Contains numerous T lymphocytes, macrophages, postcapillary venules

  
• "Domes" of Peyer's patches

  • Accumulations of connective tissue above the mantle zones of lymphatic nodules

  
• Nodule-associated epithelium of Peyer's patches

  • Located above the lymphatic nodules