1. anatomy of the immune system
Lymphoid organs:
anatomy of the immune system
Dr. Nándor Nagy
Semmelweis University,
Department of Human Morphology
And Developmental Biology,
Budapest

2. Lymphoid tissues are typically located at sites that provide a possible route of entry of pathogens and/or sites that are liable to infections.

3. Lymphoid organs primary secondary lymph nodes
spleen
Mucosa associated- Lymphoid tissue
(appendix, tonsils, etc.)
Bursa of Fabricius (only in birds!!!)
Bone marrow
Thymus
Embryonic liver

4. Hematopoiesis: Basic Concepts
All arise from a single cell type – the hematopoietic stem cell (HSC).
HSCs are pluripotent (can give rise to differentiated blood cells of all lineages – T & B & NK lymphocytes, erythrocytes, mast cells, megakaryocytes & platelets, eosinophils, basophils, neutrophils, monocytes/macrophages, DCs, etc.). 4

5. Hematopoietic cells in the embryonic liver

6. Fetal liver HSCs seed thymus, bone marrow and spleen
At about 8 wks, liver HSCs differentiate in the thymus to mature T lymphocytes (~10 wks) which populate fetal lymph nodes, spleen and gut by 12 wks and other peripheral lymphoid tissue by weeks
Bone marrow (BM) is seeded by liver HSCs by 8 weeks. B-lymphopoiesis takes place in 7 wks., then shifts to bone marrow.
Hematopoiesis from fetal BM is mainly myeloid and contributes only minimally to the blood pool throughout fetal life.
After birth, BM becomes main hematopoietic organ. 6

7. Histology of bone
B cells develop in the bone marrow which is considered a primary lymphoid organ. The bone marrow also acts as a secondary organ because it contains some antibody-secreting plasma cells as well.

8. The hematopoesis

9. T lymphocytes differentiate in thymus
The thymus is a primary lymphoid organ in that it supplies other lymphoid organs and tissues with T-lymphocytes
T lymphocytes differentiate in thymus
T cells are "educated" to distinguish self from nonself.

10. Keratinised Hassal’s body
Thymus
Keratinised Hassal’s body
Lympho-epithelial tissue. The lobular, gland-like organ is infiltrated with unmatured T-lymphocytes. The thymus becomes involuted in the adulthood, transforms to adipose tissue with islands of lymphatic cells.

11. Tonsils and adenoids are the body’s first line of defense as part of the immune system. They “sample” bacteria and viruses that enter the body through the mouth or nose, but they sometimes become infected.
Tonsils: Epithelium infiltrated by lymphatic cells and the underlying lymphatic tissue. The tonsils contain aggregetes of lymphocytes called lymphatic nodules (follicles) embedded in reticular connective tissue
Major tonsils:
The set of lymphatic tissue known as Waldeyer's tonsillar ring includes the adenoid tonsil, two tubal tonsils, two palatine tonsils, and the lingual tonsil.

12. palatine tonsil

13. palatine tonsil

14. Lymph nodes
-Lymph nodes are small, flattened, oval or bean shaped organs, which are situated in the course of the collecting lymph vessels.
-The capsule and trabeculae of lymph nodes are formed by connective tissue.
-In lymph nodes we find B- and T-lymphocytes, macrophages and reticular cells.
Vas afferens
Vas efferens

15. Appendix vermiformis

16. The spleen is an organ located in the abdomen of the human body, where it functions in the destruction of old red blood cells and holds a reservoir of blood.
The human spleen is located in the upper left part of the abdomen, behind the stomach and just below the diaphragm.
Capsule (connective tissue) 2 1 3
White pulp (lymphatic tissue
Red pulp (blood-rich area) = splenic chords + splenic sinuses
White pulp: 1.periarteriolar lymphatic sheath (PALS) – T lymphocytes
2. Malphigian nodules – B lymphocytes
3. marginal zone (rich in macrophages)

18. Antigen
presenting
cells