Thymus gland and spleen by Mohammad Mufarreh

A brief overview of the biology of the thymus, T cell development and the immunological and pathological aspects of thymus function.
The spleen is described in a nutshell.
Suitable for teaching undergraduates and reviewing for postgraduates.

1. Thymus and spleen Mohammad Mufarreh

2. Figure 20.10 Lymphoid Organs Designed to gather, destroy infectious microorganisms

3. Thymus Gland The Thymus Gland: The Heart Of The Immune System

4. Anterior view of chest showing location and size of adult thymus Anatomy

5. Thymus Structure The thymus is a primary lymphoid organ where T lymphocytes develop and undergo maturation. It consists of two lateral lobes, situated partly in the thorax above the heart, partly in the neck. Each lobe is surrounded by a capsule and is divided into lobules, which are separated from each other by strands of areolar connective tissue called trabeculae.

6. Thymus Structure Haematoxylin and eosin 1 - lobules 2 - interlobular connective tissue 3- cortex 4 - medulla 1 2 3 4

7. Thymus Structure Each follicle is organized into two compartments: The cortex , is densely packed with immature T cells, called thymocytes . The medulla where there are fewer lymphoid cells (thymocytes). It also contains nest-like bodies, concentric corpuscles of Hassall.

8. Thymus structure Cells Stromal-cell network composed of epithelial cells, dendritic cells, and macrophages. They make up the framework of the organ and contribute to the growth and maturation of thymocytes. Many of these stromal cells (e.g. Nurse Cells, cTEC , mTEC ) interact physically with the developing thymocytes.

9. Involution The thymus continues to grow between birth and puberty and then begins to atrophy; this thymic involution is directed by the high levels of circulating sex hormones. Age Mass birth about 15 grams; puberty about 35 grams twenty-five years 25 grams sixty years less than 15 grams seventy years as low as 5 grams

10. Function Development and maturation of T lymphocytes. Education of developing thymocytes by positive and negative selection. Generation of MHC restriction (positive selection). Induction of central tolerance (negative selection). Generation of diversity of TCR and immunological repertoire.

11. T Cell Development

12. T Cell Development

13. T Cell Development Color Atlas of Immunology

14. Journey Through the Thymus Nat. Imm . Rev .

15. Thymic Education The thymus induces the death of those T cells that cannot recognize antigen-MHC complexes and those that react with self-antigen– MHC and pose a danger of causing autoimmune disease. More than 95% of all thymocytes die by apoptosis in the thymus without ever reaching maturity.

16. Thymus and Disease Thymectomy Thymectomized neonate mice show a dramatic decrease in circulating lymphocytes of the T-cell lineage and an absence of cell-mediated immunity. DiGeorge Syndrome characterized by congenital thymic aplasia , or congenital deficiency of a thymus. There is an absence of circulating T cells and of cell-mediated immunity and an increase in susceptibility to infectious diseases.

17. Thymus and Disease Severe combined immunodeficiency syndromes (SCID). Autoimmune Polyendocrinopathy-Candidiasis-Ectodermal Dystrophy (APECED) caused by mutations in the Autoimmune Regulator (AIRE) gene. Myasthenia gravis. Thymomas and Lymphomas. HIV/AIDS.

18. Thymus Paradox Surgical removal or involution of the thymus does not result in T cell immunodeficiency !!!!!!!!!!!!!!??? Sufficient T cells are generated during fetal life that are long-lived and can perform homeostatic proliferation throughout the lifetime of the patient. Secondary thymi. Extra-thymic development of T-cells.

19. Spleen The Belly of The Beast A living Hell for intruders

20. Spleen The spleen plays a major role in mounting immune responses to antigens in the blood stream. It is a large, ovoid secondary lymphoid organ situated high in the left abdominal cavity. While lymph nodes are specialized for trapping antigen from local tissues, the spleen specializes in filtering blood and trapping blood-borne antigens; thus, it can respond to systemic infections.

21. Spleen

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

23. Spleen

24. Spleen red pulp: consists of a network of sinusoids populated by macrophages and numerous red blood cells (erythrocytes) and few lymphocytes; it is the site where old and defective red blood cells are destroyed and removed. white pulp: surrounds the branches of the splenic artery, forming a periarteriolar lymphoid sheath (PALS) populated mainly by T lymphocytes.

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