Primary Lymphoid organs

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Thymus gland.
Thymus gland is a very important primary lymphoid organ. It is an important site of T-lymphocyte growth and maturation. Loss of the thymus at an early age as in DiGeorge Syndrome or surgical removal will results in severs immunodeficiency and a high susceptibility to infection.
Location of Thymus gland...

It located in the upper anterior portion of the chest cavity, just behind the sternum, superior to heart.


It develops from ectoderm derived from the third and fourth pharyngeal pouch, Reach its maximum size just prior to birth, Continuous to grow till age of puberty around 12th year.
Structure of Thymus gland.
It is formed of two lobes, each lobes is surrounded by capsule and is divided into lobules. Lobules are separated from each other by connective tissues. Each lobule is divided into outer cortex and inner medulla. Outer cortex contains numerous immature lymphocytes (thymocytes). Inner medulla contains mature T lymphocytes.

The cortex is stain more densely with haematoxylin and eosin. Where the medulla is less densely stained with haematoxylin and eosin. The thymus reaches is peak size around puberty, after which it shrinks or involutes. In adults it is composed mostly of fat cells and connective tissue.

Numerous different cell types can be found in the thymus:

1. Epithelial cell

  1. Cortical epithelial cells. They provide structure and secrete factors that are essential for T cell development.

  2. Nurse cells. These are another type of cortical epithelial cell and can be seen in close contact surrounding developing thymocytes. They express class I and class II MHC on their surface.

  3. Medullary epithelial cells. Theses provide structure for the medulla and recent evidence suggests they play an important part in T cell tolerance.

Deep in the medulla they are also aggregated into Hassall's corpuscles (of unknown function), they appear very early in life and contain debris, nonfunctional degenerative material.
2. Macrophages. These are found in both the cortex and medulla, although they are more numerous in the medulla. They are class II MHC negative and play an important role in phagocytosis of thymocytes that have died by apoptosis during development.

3. Dendritic cells. These are another type of bone-marrow derived cells. They express class II MHC as well as class I MHC.

4. Thymocytes. These are the most abundant cells in the thymus and consist of T cells in various stages of development.

Function of Thymus gland.
1. Primary function of thymus is the production of thymic lymphocytes.

2. It is the major site for lymphocyte proliferation in the body.

3. Maturation and selection of T cell takes place in thymus.

A. maturation of T-cell. In thymus gland the T-cell develop their specific T cell markers, including TCR, CD3, CD4 or CD8 and CD2. during this Maturation stage the T-cells must be successfully rearrange pairs of genes that encode a heterodimeric T-cell receptor (α β or γ δ).

B. Thymic education and T-cell selection. T-cells education take place within thymus gland through two important process of selection 1) positive selection of T lymphocytes: T cell precursors that express receptors capable of binding with self-MHC. These cells are allowed to proliferate and survive. 2) Negative selection of T lymphocytes. Any developing thymocytes that have high affinity for self-antigen in association with MHC molecules will die within the thymus gland. This process the body will maintain the self-tolerence and prevent the autoimmune reaction and subsequently prevent the occurrence of autoimmune disease.

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