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Human Structure > Histology: Lymphatic System > Flashcards

Histology: Lymphatic System Flashcards

1
Q

Primary Lymhpoid Organs

A

Sites where lymphocytes are made and undergo antigen independent differentiation

  • Bone Marrow
  • Thymus
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2
Q

Secondary Lymphoid Organs

A

Lymphocytes undergo antigen-dependent differentiation

  • Lymph Nodes
  • Spleen
  • MALT
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3
Q

Lymphocytes

A
  • 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

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4
Q

Lymphoid Tissue

A
  • 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
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5
Q

Diffuse Lymphoid Tissue

A
  • Free lymphocytes, no organization
    • ​Not encapsulated
  • Lamina propria of mucous membranes
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6
Q

Nodular Lymphoid Tissue

A
  • Lymphocytes arranged into spherical masses
  • Not encapsulated
  • Primary Nodules (non-activated)
  • Secondary Nodules (activated)
    • Germinal Center
      • Actively dividing lymphocytes
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7
Q

Mucosa Associated Lymphoid Tissue

A
  • 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
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8
Q

Tonsils

A
  • Partially encapsulated lymphoid tissue at entrance of pharynx
  • Contains diffuse and nodular lymphoid tissue
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9
Q

Peyer’s Patches

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

Appendix

A
  • Mucosa and submucosa almost entirely filled with lymphoid tissue
    • Obscures glands
    • Lymphoid tissue decreases with age
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11
Q

Lymph Nodes

A
  • 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
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12
Q

Lymph Flow through Lymph Node

A
  • 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
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13
Q

Lymphocyte Circulation

A
  • Most lymphocytes enter lymph nodes via HEVs in paracortex
    • HEVs have receptors for antigen-primed lymphocytes
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14
Q

Spleen

A
  • 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)
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15
Q

Splenic Sinuses

A
  • Sinusoidal capillaries comprised of stave cells (elongated endothelial cells)
    • Discontinuous basal lamina and reticular fibers
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16
Q

Splenic Circulation

A

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
17
Q

Thymus

A
  • 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
18
Q

Thymic Medulla

A
  • Mature T lymphocytes
  • Thymic epithelial cells
  • Hassall’s Corpuscles
    • Concentrically arranged thymic epithelial cells
    • May secrete factors related to T-Cell differentiation
19
Q

Blood-Thymus Barrier

A
  • 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
20
Q

T Cell Education

A

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

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