Histology: Lymphatic System

Question 1

Primary Lymhpoid Organs

Answer 1

Sites where lymphocytes are made and undergo antigen independent differentiation

• Bone Marrow
• Thymus

Question 2

Secondary Lymphoid Organs

Answer 2

Lymphocytes undergo antigen-dependent differentiation

• Lymph Nodes
• Spleen
• MALT

Question 3

Lymphocytes

Answer 3

• Arise from stem cells in bone marrow

B Lymphocytes

• Mature in bone marrow
• Humoral immunity
• Mature in plasma cells (secrete antibodies)

T Lymphocytes

• Mature in thymus
• Cell-mediated immunity
• Helper T Cells
• Cytotoxic T Cells

Natural Killer Cells

• Innate immunity

***B and T lymphocytes cannot be differentiated histologically

Question 4

Lymphoid Tissue

Answer 4

• Reticular CT filled with large numbers of lymphocytes
  
  • Reticular cells (secrete reticular fibers)
    
    • No reticular cells/fibers in thymus
  • Antigen-presenting cells (macrophages)
  • Plasma cells

Question 5

Diffuse Lymphoid Tissue

Answer 5

• Free lymphocytes, no organization
  
  • ​Not encapsulated
• Lamina propria of mucous membranes

Question 6

Nodular Lymphoid Tissue

Answer 6

• Lymphocytes arranged into spherical masses
• Not encapsulated
• Primary Nodules (non-activated)
• Secondary Nodules (activated)
  
  • Germinal Center
    
    • Actively dividing lymphocytes

Question 7

Mucosa Associated Lymphoid Tissue

Answer 7

• Associated with GI, respiratory, and genitourinary tracts
  
  • ​GALT in GI tract, tonsils, peyer’s patches, appendix
  • BALT in respiratory tract
• >70% body’s immune cells
• Diffuse and nodular lymphoid tissue

Question 8

Tonsils

Answer 8

• Partially encapsulated lymphoid tissue at entrance of pharynx
• Contains diffuse and nodular lymphoid tissue

Question 9

Peyer’s Patches

Answer 9

• Aggregations of lymphoid nodules in ileum
• Each patch contains 10-200 nodules

Question 10

Appendix

Answer 10

• Mucosa and submucosa almost entirely filled with lymphoid tissue
  
  • Obscures glands
  • Lymphoid tissue decreases with age

Question 11

Lymph Nodes

Answer 11

• Bean-shaped, encapsulated lymphatic organs
  
  • Filter lymph

Capsule

• Dense CT
• Trabeculae extend into node

Cortex

• Lymphatic nodules
• Reticular cells/fibers
• Macrophages, APCs, B lymphocytes

Paracortex

• No nodules
• T Lymphocytes
• Contains high endothelial venules

Medulla

• Medullary Cords (darker areas)
  
  • ​B and T lymphocytes
• Medullary Sinuses
  
  • Spaces that contain lymph

Question 12

Lymph Flow through Lymph Node

Answer 12

• Lymph travels into node via afferent lymphatic vessels
• Lymph enters subcapsular sinus
  
  • goes to trabecular sinuses
• Enter medullary sinuses
• Exits via efferent lymphatic vessel at hilum

Question 13

Lymphocyte Circulation

Answer 13

• Most lymphocytes enter lymph nodes via HEVs in paracortex
  
  • HEVs have receptors for antigen-primed lymphocytes

Question 14

Spleen

Answer 14

• Filters blood
• Encapsulated: has dense CT capsule
  
  • Trabecular extend into organ
• Splenic Pulp
  
  • Red Pulp
    
    • ​Destruction of old/damaged RBCs
    • Splenic Cords (lots of lymphocytes)
    • Splenic Sinuses
  • White Pulp
    
    • 20% of spleen parencyhma
    • Splenic nodules (B lymphocytes) with _central artery_ (surrounded by periarterial lymphatic sheaths with T lymphocytes)

Question 15

Splenic Sinuses

Answer 15

• Sinusoidal capillaries comprised of stave cells (elongated endothelial cells)
  
  • Discontinuous basal lamina and reticular fibers

Question 16

Splenic Circulation

Answer 16

Splenic a –> trabecular a. –> central a. –> splenic nodule –> penicillar arteriole

Closed Circulation:

• Capillaries from penicillar arterioles connect directly to splenic sinuses, then venous system
• Blood cells stay in endothelial lined space

Open Circulation

• Capillaries from penicillar arterioles open into splenic cords of red pulp
  
  • Good RBCs re-enter sinuses by passing between stave cells

Question 17

Thymus

Answer 17

• Primary lymphoid organ
  
  • ​Site of T lymphocyte maturation/differentiation
• Located in anterior mediastinum
  
  • Large in children, replaced by adipose in adults
• DiGeorge Syndrome: absence of thymus

Capsule

• Vascularized CT with trabeculae extending into parenchyma forming lobules

Cortex

• T lymphocytes (thymocytes)
• Thymic epithelial cells
• Macrophages

Question 18

Thymic Medulla

Answer 18

• Mature T lymphocytes
• Thymic epithelial cells
• Hassall’s Corpuscles
  
  • Concentrically arranged thymic epithelial cells
  • May secrete factors related to T-Cell differentiation

Question 19

Blood-Thymus Barrier

Answer 19

• Protects developing T cells from contact with antigens in blood
• Endothelium of continuous capillaries surrounded by complete basal lamina
  
  • Macrophages residing in perivascular CT
  • Thymic epithelial cells in underlying basal lamina
• Breakdown of this barrier would result in tolerance of foreign antigens

Question 20

T Cell Education

Answer 20

Positive Selection

• Cortex
• Immature T Cells with self and foreign antigens
  
  • If either is recognized, cell survives (if not, cell dies)
  • Check to see if T Cell receptor is functional

Negative Selection

• Medulla
• T Cells that recognize self are eliminated
• Ensures T cells will not attack body’s own cells

98% of T cells fail

Surviving T cells differentiate to cytotoxic T cells/helper T cells