Role of Thymus in immunity

Under Supervision:

Dr.Gomaa Abdelrahim Abdullalim

Abdullah adehim alenaziMohammd wdad alenazi

Kingdom of Saudi ArabiaMinistry of Higher EducationMajmaah UniversityFaculty of Science in AlZulfi

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

The Thymus:Gross Specimen

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.

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

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.

Medulla Cortex

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.

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!

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.

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

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.

Impact of thymectomyPossible 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.