1. Role of Thymus in immunity Under Supervision: Dr.Gomaa Abdelrahim Abdullalim Abdullah adehim alenazi Mohammd wdad alenazi Kingdom of Saudi Arabia Ministry of Higher Education Majmaah University Faculty of Science in AlZulfi

2. What is the thymus? Is a primary lymphoid organ, which are critical cells of the adaptive immune system. Thymus Location In the upper parts of thorax, behind the sternum

3. The Thymus: Gross Specimen

4. Thymic Structure Connective tissue capsule, trabeculae. Both contain blood vessels, efferent lymphatic vessels, and nerves. Trabeculae demarcate thymic lobules. Parenchyma is made up of both a cortex and medulla.

5. Development of Thymus Changes with age Overall Size birth 10-15 g puberty 30-40 g after puberty - involution Replaced by adipose tissue middle-aged 10 g

6. Thymus: Cortex and Medulla The cortex stains darkly basophilic because there lots of small lymphocytes with densely stained nuclei. The medulla stains light because it has fewer lymphocytes with more cytoplasm. MedullaCortex

7. Thymus: 3 cell types Epithelioreticular cells: large, pale, and stellate. (They are not reticular fibers!) Thymocytes: Densely- packed developing T cells derived from stem cells made in the bone marrow. Macrophages: phagocytose T cells found in the cortex and near the cortico-medullary junction, cells that react too strongly with self-antigens. They also present antigen to lymphocytes.

8. Hassall’s Corpuscles Concentrically arranged keratinizing and degenerating epithelioreticular cells and macrophages. Function is poorly understood (thymic hormones?) Found in the medulla. Instantly signify the thymus!

9. Thymus: The Education of T cells The thymus is the location where thymocytes mature and proliferate. Thymocytes undergo positive and negative selection. Schematic: multipotential stem cells enter thymus via postcapillary venule  positive selection in cortex  negative selection in medulla  naive T cells exit thymus from medulla and enter blood circulation.

10. Blood-Thymic Barrier Separates developing T cells from blood (prevents T cells from recognizing foreign proteins as “self”). Components (from outside  inside) Capillary endothelium Endothelial basal lamina Perivascular connective tissue sheath (and macrophages!) Basal lamina of epithelioreticular cell Epithelioreticular sheath

11. Functions of thymus in immunity Lymphocytes produced in the bone marrow migrate to the thymus and are processed. These T-cell undergo development, differentiation and clonal expansion. These thymus processed lymphocytes are responsible for cell mediated immunity. It is the site where T-lymphocytes acquire their immunological incompetence to be disseminated later in other lymphoid tissue. Produce hormones which Activates the immune system by activating the T-Cells (T-Killer Cells; T-Helper Cells; T-Memory Cells). Thymic positive selection for thymocytes bearing receptors capable of binding self-MHC molecules, which results in MHC restriction. Cells that fail positive selection are removal by apoptosis. Thymic negative selection that eliminates thymocytes bearing high-affinity receptors for self-MHC molecules alone or self-antigen presented by self-MHC, which results in self-tolerance.

12. Impact of thymectomy Possible thymectomy complications include respiratory failure during or in the first few days or weeks after the procedure, which is a risk mostly for older men and women. There is also a risk of infection at the wound site, and a very small risk of nerve damage during the operation. Any symptoms of infection, such as chills, fever, swelling, redness, or pain at the infection site should get prompt medical attention. Symptoms of respiratory problems, such as difficulty breathing, should also be treated promptly.