Presentation on the Overview of lymphoid system including histology slides. This will be useful for everyone those who are interested in & specially for self reviewing.

1. HistologyLymphoid System by Yapa Wijeratne Faculty of Medicine University of Peradeniya Sri Lanka

2. You have two types of blood cells: 1. Red Blood Cells (we call these 'Erythrocytes') 2. White Blood Cells (we call these 'Leukocytes')

3. Erythrocytes These are pretty straight forward. Look at the Erythroid Series. Leukocytes The leukocytes are part of our immune defense system. There are three types of leukocytes: 1. Granulocytes (These are also called 'Myeloid Cells.') 2. Monocytes 3. Lymphocytes Some sources combine the monocytes and lymphocytes into one category and just call them all 'mononuclear leukocytes.'

4. Granulocytes inlcude the neutrophils, eosinophils, and basophils. (It shouldn't surprise you to learn that their cytoplasm is often filled with granules.) These are the work horses of acute inflammation (and other processes).

5. In pathology and immunology, you'll call neutrophil, the PMN (polymorphonuclear leukocyte). Eosinophils are involved in allergic reactions & parasitic infections.

6. Mast Cell Another cell to mention here (although it is NOT a granulocyte) is the Mast Cell. It's very similar to the basophil: both release histamine (and other mediators). Some think that the mast cell is derived from the basophil. Just remember that the basophil circulates and the mast cell is found in peripheral tissues. Other than that, they are quite similar.

7. Monocytes include the monocyte and the macrophage. The monocyte circulates in the blood until it receives the signal to extravasate into the peripheral tissue. Once in the tissue, it matures into the macrophage. It can also mature even further into other cells, but that is beyond the scope of this course. The monocyte/macrophage is the work horse of chronic inflammation.

8. Lymphoctyes are often overlooked when we consider blood, but they are white blood cells. Indeed, they originate in the bone marrow and are derived from the same stem cell as the rest of the erythrocytes and leukocytes. These are the T-cells and B-cells that direct the immune system and produce antibodies, respectively. They are the central cells in our cell-mediated and humoral (antibody) defense mechanisms. Also keep in mind that the B-cell can mature into the plasma cell.

9. If you remember nothing else... Know that as a rule of thumb, bacterial infections cause granulocytosis and viral infections cause lymphocytosis. There are exceptions to this, but this is a very basic (and important) concept. You'll learn more about why this is in immunology. Spend some time learning the pathways from the original stem cell and realize that it gives rise to all these cells

10. Infant Thymus The lobules of the infant thymus are separated by connective tissue septa.

11. Infant Thymus Notice that the medulla (M) tends to stain lighter than the cortex (C).

12. Hassall's Corpuscles The thymus has no germinal centers like many other lymphoid tissues, but it does have distinguishing Hassall's corpuscles (arrows) comprised of multiple layers of epitheloid cells.

13. Adult Thymus The post-pubescent thymus involutes and is characterized by areas of adipose tissue (A).

14. Palatine Tonsil The palatine tonsil is partially encapsulated in connective tissue. Look for the lighter-stained germinal centers (GC).

15. Pharyngeal Tonsil The pharyngeal tonsil is distinguished from the palatine by the presence of pseudostratified columnar epithelium (arrows).

16. Peyer's Patch Don't forget the GI tract! The Peyer's Patches are visible with the naked eye. Notice the germinal center where B-cells proliferate. These are a major source of antibody production.

17. Appendix The vermiform appendix houses a large number of lymphocytes and lightly-stained germinal centers.

18. Spleen The distinctive red pulp (RP) and white pulp (WP) identify the spleen. No you're not color blind ... histologists call that blue tissue "white". (Now that you know a little about the immune system, can you guess why?)

19. Spleen The red pulp (RP) is composed of open sinusoids containing blood. The white pulp (WP) contains lymphocytes.

20. Lymph Node The lymph node filters both lymph and blood. Lymph enters the subcapsular sinuses (SS) and exits the hilar region whereas blood both enters and exits.

21. Lymph Node Find the germinal centers in the darkly stained cortex. Notice also the connective tissue capsule that surrounds the node.

22. Lymph Node Lymph first enters the sub-capsular sinus (SS) and then percolates through various trabeculae (T) to the medulla.

23. Lymph Node Once in the medulla, lymph is channeled through medullary cords (MC) to the hilum.

24. Plasma Cells Don't forget that plasma cells are derived from B lymphocytes. These cells are antibody factories. They produce thousands of antibodies per second for a few days and then they die.

25. Note: This presentation was prepared using photographs & content from various web sites & textbooks on the assumption of fair usage for studying (specially teaching & self reviewing ) & is for NON-COMMERCIAL purposes.