Lymphoid System I & II

1. 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 response20 – 50% WBC’s are lymphocytes

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

10. ImmunoblastsForm from T or B lymphocytes when stimulated by Antigen • Antigen -presenting cellsMonocyte-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. ThymusLocated 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. Tonsils1. Palatine 2. Lingual 3. Pharyngeal

38. 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 nodeHEV with cuboidal endothelium Lymphocytes pass from blood into lymph node