1. Dr. Jack L. Haar Department of Anatomy and Neurobiology
Lymphoid System I & II
Dr. Jack L. Haar
Department of Anatomy and Neurobiology

4. Overview of Immune System
Functions
Provides immune surveillance and defense
Provides immune tolerance (distinguishes self from non-self)
Absorbs lipids
Maintains fluid balance by returning tissue fluid and lymphocytes to the blood

5. Overview continued Concept of immunity
Immune system provides a way to recognize “self” from “non-self”
An individual mounts an immune response to foreign material
If person survives an attack by foreign material (infection), immunity may result, meaning the foreign material is “remembered”
Thus immunity has specificity and memory

6. Overview continued Terminology
Antigen: a foreign (non-self) substance. Could be bacterial, tumor cells, transplanted cells or virus-infected cells
Antibody: circulating protein in blood plasma (immunoglobulin) that interacts with a specific antigen
Humoral immunity: antibodies against antigen circulating in blood stream; produced by plasma cells derived from B lymphocytes
Cellular (cell-mediated) immunity: Immunocompetent cells contact, react against, and destroy antigen; mediated by T lymphocytes

7. Major lymphoid organs and tissues

8. Cells of the Immune response 20 – 50% WBC’s are lymphocytes

9. A. 35% of circulating lymphocytes are B lymphocytes or B cells
A. 35% of circulating lymphocytes are B lymphocytes or B cells Become Plasma cells or B memory cells B. 65% of circulating lymphocytes are T lymphocytes or T cells Become Helper, Killer, or T memory cells

10. Immunoblasts Form from T or B lymphocytes when stimulated by Antigen
Antigen -presenting cells Monocyte-macrophage derived. Process antigen to present to lymphocytes
Includes macrophages, epidermal Langerhans cells, dendritic cells of lymphoid organs, and epithelial cells of thymus

11. Lines of Defense

13. IV. Thymus Located in mediastinum

14. Thymus undergoes involution with age
Is influenced by adrenal cortical steroids and radiation
Thymus is a bilobed organ

16. Stroma formed from thymic epithelial cells with bundles of tonofibrils and linked by desmosomes
Thymocytes develop from HSC from yolk sac (mesoblastic), fetal liver (hepatic), and bone marrow (myeloid) phases of development
Thymus is not exposed to external environment

17. Histology Dense irregular CT capsule Septa form lobules
Cortex and medulla
Stroma formed from thymic epithelial cells

18. Cortex Thymic epithelial cells
Large numbers of thymocytes (lymphocytes)
Macrophages

19. Medulla
Thymocytes are larger and fewer in number than in cortex Only 5% of thymocytes are in the medulla
Hassall’s corpuscles form from the stromal cells Keratohyaline granules

20. Vessels enter and leave through the capsule and follow the septa
Blood thymus barrier

21. Function of the thymus Thymus is seeded with HSC’s
Thymocytes proliferate in cortex
Self-reactive thymocytes are eliminated
Non-self reactive cells migrate to medulla
Cells enter blood stream and migrate to secondary lymphoid organs
They occupy T-dependant areas of lymphoid organs where they nest, divide, and mature
The thymus is an endocrine organ and responds to many hormones, viz. ACTH, GH, and sex hormones
Thymectomy at birth impairs immune function

22. Bursa of Fabricius
Functions in birds as primary lymphoid organ giving rise to B-lymphocytes

23. Bone marrow
Functions in mammals as primary lymphoid organ giving rise to B-lymphocytes

24. Lines of Defense

25. Outer Line of Defense - MALT

26. MALT GI tract Respiratory tract Genito-urinary tract
Diffuse lymphatic tissue
Lamina propria
Reticular fibers
Lymphocytes
Primary lymphatic nodule

27. Secondary Lymphatic nodule
Germinal center
B-dependant spherical area
Cap of tightly packed small lymphocytes
Not encapsulated by C.T.

28. Germinal center Oval pale-staining area
Contains dividing cells and macrophages

29. Solitary nodules
Not encapsulated by C.T.
May be primary or secondary

30. High endothelial venules
Associated with diffuse lymphatic tissue
Allow lymphocytes to escape from vessel

31. Locations of aggregates of nodules
Peyer’s patches
Typhoid nodules
Appendix

32. Typhoid Fever - a bacterial illness caused by Salmonella typhi

33. Appendix

34. Tonsils 1. Palatine 2. Lingual 3. Pharyngeal

38. Dr. Jack L. Haar Department of Anatomy and Neurobiology
Lymphoid System II
Dr. Jack L. Haar
Department of Anatomy and Neurobiology

39. Lines of Defense

40. Inner Line of Defense

43. Lymphatic Vessels Lymphatic capillaries Lacteals for lipid
Valves direct flow
Asymmetrical system
Edema

44. Lymphatic filariasis, also known as elephantiasis, is best known from dramatic photos of people with grossly enlarged or swollen arms and legs. The disease is caused by parasitic worms, including Wuchereria bancrofti, Brugia malayi, and B. timori, all transmitted by mosquitoes. Lymphatic filariasis currently affects 120 million people worldwide, and 40 million of these people have serious disease.

45. Lymph node Histology Distributed along vessels Filter lymph
Produce lymphocytes
Dense CT capsule
Afferent lymphatics on convex surface
Reticular fiber framework
Subcapsular sinus
Lymphoid nodule in
cortex

46. Hilus
Blood vessels enter and leave
Efferent lymphatics leave node

47. Cortex of lymph node Lymphoid nodules and diffuse lymphatic tissue
Lymphatic sinuses
Subcapsular
Cortical

49. Medulla of lymph node
Medullary cords
Medullary sinuses

50. Blood vessels follow trabeculae into node
Blood vessels follow trabeculae into node HEV with cuboidal endothelium Lymphocytes pass from blood into lymph node

51. B and T-dependant areas
Outer cortex and medullary cords are B-dependant
Inner cortex (paracortical area) is T-dependant

52. Cells found in a lymph node
Lymphocytes of all sizes
Plasma cells
Macrophages (dendritic or reticular cells
Antigen presenting cells

53. Functional significance of Lymphatic Tissue
Lymph flowing through lymph vessels is filtered by at least one lymph node
Macrophages in lymph node remove 99% of antigen in lymph
1% of antigen passes through a lymphoid nodule
The antigen stimulates formation of germinal centers
Long-lived lymphocytes present in blood re-enter lymph nodes via HEV to stimulate immune response
Tumor cells may be trapped in lymph nodes

54. Lines of Defense

56. Third Line of Defense

58. Spleen Filters blood Largest lymphoid organ Non-vital organ
Dense CT capsule and trabeculae
Covered by mesothelium
Reticular fiber framework

59. White pulp
Periarterial lymphoid sheath (PALS): T-dependant
Germinal centers: B-dependant
Marginal zone
Red pulp
Splenic (Billroth) cords
Venous sinusoids, arterioles, venules

60. Central artery (arteriole) with PALS and Germinal center

61. Central artery (arteriole) with PALS and Germinal center

62. Red pulp: Splenic cords and venous sinusoids

64. Circulation of blood through the spleen
Splenic artery
Trabecular arteries
Central artery (arteriole)
Pulp artery
Penicillar arteriole
Venous sinuses
Veins
Trabecular veins
Splenic vein

65. Functions of the Spleen
Graveyard of RBC’s
Filters blood to remove bacteria, viruses and inert particles
Blood cell production
Lymphocytes in the white pulp
Plasma cells and monocytes in the red pulp
Erythrocytes in fetus and with certain infections