1. THE LYMPHOID SYSTEM
Kristina C. Erasmo, M.D.

2. Defense System of the Body
Non-Immune
Immune

3. Non-Immune Defense System
INFLAMMATION
Mediated by:
Neutrophils
Eosinophils
Basophils
NK cells
Monocytes and macrophages

4. Immune Defense System
Lymphocytes – principal effector cells

5. Lymphoid System
Refers to the tissues and organs that participate in the immune defense system
Components are distributed in various areas of the body

6. LYMPHOID SYSTEM Non-Encapsulated Lymphoid Tissue
Encapsulated Lymphoid Organs
Tonsils

7. LYMPHOID SYSTEM Non-Encapsulated Lymphoid Tissue
Diffuse lymphoid tissue
Lymphoid nodules (lymphoid follicles)
Encapsulated Lymphoid Organs
Lymph Node
Thymus
Spleen

8. LYMPHOID SYSTEM
Tonsils
Palatine
Pharyngeal
Lingual
Tubal

9. LYMPHOID SYSTEM Non-Encapsulated Lymphoid Tissue
Diffuse lymphoid tissue
Lymphoid nodules (lymphoid follicles)

10. Diffuse Lymphoid Tissue
Histologic features:
Stroma composed of network of reticular fibers and reticular cells
Spaces occupied primarily by lymphocytes
Some macrophages found

11. Diffuse Lymphoid Tissue
Location:
Practically in all CT in the body
Most prominent in lamina propria and submucosa of GIT, respiratory tract

12. Diffuse Lymphoid Tissue (Stomach)

13. Diffuse Lymphoid Tissue
Types:
Loose lymphoid tissue – lymphocytes far apart
Dense lymphoid tissue – lymphocytes tightly packed

14. Lymphoid Nodule LYMPHOID FOLLICLE
Lymphoid tissue where clustered lymphocytes form discrete masses or lumps
Usually interspersed in areas of diffuse lymphoid tissue

15. Lymphoid Nodule Location: Lamina propria of GIT and respiratory tracts
Spleen
Lymph nodes
Tonsils

16. Lymphoid Nodule Types based on arrangement:
Solitary nodule – occur singly
Aggregates – Peyer’s patches (submucosa of the ileum)

17. Peyer’s Patch

18. Peyer’s Patches

19. Lymphoid Nodule Types based on structure:
Primary nodule – cells evenly distributed throughout nodule, no mitotic figures seen
Secondary nodule (germinal centers) – 2 distinct regions

20. Lymphoid Nodule Secondary nodule
Germinal center (reaction center) – pale, central
Forms the core of the nodule
B lymphocytes proliferate and differentiate following exposure to antigen
Many mitotic figures
Very few T lymphocytes
Corona – darker, peripheral

21. Secondary Lymphoid Follicle

22. Secondary Lymphoid Follicle (Lymph Node)

23. LYMPHOID SYSTEM
Encapsulated Lymphoid Organs
Lymph Node
Thymus
Spleen

24. Lymph Node Encapsulated, bean-shaped organ Location:
Popliteal, inguinal, axillary regions
Sides of neck
Along abdominal vessels
In mesentery

25. Lymph Node

26. Lymph Node
Hilus – the indented area of the lymph node, where blood vessels enter and leave the organ
Efferent lymphatic vessels leave the lymph node on the hilar area
Afferent lymphatic vessels enter the lymph node on the convex side

27. Lymph Node

29. Histologic Features of Lymph Node
Capsule – dense connective tissue covering
Trabeculae – incompletely subdivide the organ into compartments
Reticular cells and fibers – supporting meshwork

30. Lymph Node

32. Histologic Features of Lymph Node
Parenchyma:
Cortex – outer
Outer portion – composed of lymphoid nodules (primary and secondary), mostly B lymphocytes
Inner – dense lymphoid tissue, no lymphoid nodules, mostly T lymphocytes
Medulla – inner, paler-staining, contain medullary cords

33. Lymph Node

34. Lymph Node

35. Spleen Largest lymphoid organ (7 x 12 cm)
Found in the upper left quadrant of the abdominal cavity
Hilus – where splenic vessels enter and leave the spleen

37. Spleen
Functions:
Contain macrophages – destroy foreign substances, microorganisms, abnormal cells in blood
Removes and destroys old RBCs and platelets from circulating blood
Recycles iron contain in the RBCs

38. Spleen Functions: Storage area for blood
Where lymphocytes proliferate and differentiate into different types after being stimulated

39. Histologic Features of Spleen
Enveloped by peritoneum that blends with its capsule (mesothelium)
Trabeculae – divide into compartments
Reticular cells and fibers – supporting framework

40. Spleen

41. Histologic Features of Spleen
Splenic pulp – parenchyma of the spleen
White pulp
Red pulp
Marginal zone

42. Histologic Features of Spleen
White pulp
Consists of lymphoid nodules embedded in dense lymphoid tissue
Lymphoid nodules – B lymphocytes
Dense lymphoid tissue – T lymphocytes

43. Histologic Features of Spleen
Red pulp
Bulk of the parenchyma
Consists of large blood-filled sinusoids with strands of lymphoid tissue in-between (Billroth’s cords, splenic cords)

44. Histologic Features of Spleen
Marginal zone
Poorly-delineated transitional area between red and white pulp
Contains lymphocytes and plasma cells

45. Spleen

46. Red Pulp (Spleen)

47. Thymus Located in the superior mediastinum
Composed of two pyramidal lobes fused together

49. Thymus
Where T cell stem cells proliferate, differentiate, and transform into immunologically competent T lymphocytes
Devoid of B lymphocytes

50. Histologic Features of Thymus
Capsule – thin loose connective tissue
Trabeculae
Reticular cells – supporting meshwork

51. Histologic Features of Thymus
Cortex – peripheral, darker-staining region
Medulla – central, lighter-staining region
Hassall’s corpuscles (Hassall’s bodies, thymic corpuscles)
Composed of hyaline core surrounded by layers of flattened epithelioid cells

52. Thymus

53. Thymus

54. Tonsils
Lymphoid organs that form a ring (Waldeyer’s ring) underneath the epithelium around the entrance to the respiratory and digestive passages

58. Tonsils Histologic feature:
Dense lymphoid tissue embedded with lymphoid nodules
Types:
Palatine, lingual, pharyngeal, tubal tonsils

59. Palatine Tonsils
Located in the lateral aspect o the oropharynx (one on each side)
Covered by stratified squamous non-keratinized epithelium
Tonsillar crypts – deep invaginations of the epithelium (contain dead epithelial cells, lymphocytes, etc)

62. Lingual Tonsils
Several discrete masses of lymphoid tissue located in the dorsum of the tongue
Covered by stratified squamous non-keratinized epithelium
May have single broad deep crypt into which ducts of mucus-secreting glands open

64. Pharyngeal Tonsils
Central area of the posterior and superior nasopharyngeal wall
Covered by ciliated pseudostratified columnar epithelium (“respiratory epithelium”)
Some areas covered with stratified squamous non-keratinized epithelium
Does not form crypts but form shallow folds
Adenoids – enlarged pharyngeal tonsils

66. Tubal Tonsils
Masses of lymphoid tissue in the nasopharynx near the openings of the eustachian tube
Some believe they are simple extensions of the pharyngeal tonsils
Covered by respiratory epithelium