Lymphoid System

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Lymphoid System
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Lymphoid System

• Functions
• Immunologically mediated defense against foreign
  antigens (often infectious)
• Intimately associated with vascular system

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Components of lymphoid system

• Lymph and lymph nodes
• Lymph fluid scavenged from the intersitium
• Collected in thin walled lymphatic vessels
• Returned to general circulation
• Contains leukocytes
• Filtered through small specialized organs (lymph
  nodes)

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Additional lymphoid Tissues

• Thymus
• Spleen
• Tonsils
• Mucosal associated lymphoid tissue (MALT)
• Gut-associated (GALT)
• Bronchial-associated (BALT)
• Bursa of Fabricius (avian)

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Lymph node

• Locations
• Distributed along regional lymphatics
• Aggregates in
• Neck, axillae, groin, lung hilus
• At sites were major lymphatic vessels converge

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Lymphatic vessels

• Thin walled
• Originate as blind open vessels
• Have valves to allow one-way flow
• Muscular activity is necessary for efficient flow

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Lymph node

• Functions
• Filtration of lymph
• Initiation of immune response
• Activation and proliferation of lymphocytes

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Lymph node

• Cell types
• Lymphocytes and plasma cells
• Accessory cells
• Macrophages
• Follicular dendritic cells
• Interdigitating dendritic cells
• Stromal cells

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Lymph node

• Circulation of lymphocytes and lymph
• Enter and exit via blood or lymph
• If able to respond to any antigen presented
  stay and differentiate
• If unable to respond exit via efferent
  lymphatics

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Lymph node architecture
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Lymph node architecture
HEV specialized Post-capillary
venules. Vascular addressins Special surface
markers Cuboidal endothelium
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Compartments

• Sinuses
• Lined by endothelium and macrophages
• Contain lymphocytes, macrophages, langerhans
  cells
• Site of filtration and antigen processing

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Lymph node capsule
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Lymph node - subcapsular sinus
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Compartments

• Vascular system
• Entry/exit site for lymphocytes and dendritic
  cells

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Postcapillary venules
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Compartments

• Interstitial tissue
• Cortex
• Paracortex
• Medullary cords
• Site of maturation and proliferation of
  lymphocytes

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Lymph node
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Lymph node reticulum stain
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Lymph node
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Lymphoid follicle
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Lymphoid follicle immunohistochemistry
T-cell marker
B-cell marker
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Follicular dendritic cells
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Antigen processing cell nucleus
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Lymph node medullary cords
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Medullary cords reticulum stain
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Lymph node medullary sinuses
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Plasma cell
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Pig lymph node

• Inverted structure
• Cortex is internal
• Medullary tissue is external
• Still has diffuse paracortical tissue and nodular
  (follicular) tissue

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Compartmentalized Responses

• Cell-mediated responses will expand the
  paracortex
• Humoral responses will expand the cortex

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Hemal nodes

• Associated with blood vessels not lymphatics
• Are blood filled
• Have capillary plexuses with post-capillary
  venules similar to HEV
• Have cortex and medulla (nodular and diffuse
  tissue
• Are little spleens