The thymus gland is a small organ behind the breastbone that plays an important role in the immune system. During development, it helps train T cells to fight infections and cancer. After puberty, it begins to shrink and be replaced by fat. The thymus contains different cell types and acts as both a lymphatic and endocrine organ, producing hormones that help T cells mature. Disorders of the thymus can impact immunity and increase autoimmune disease risk.

1. ANATOMY AND PHYSIOLOGY OF
THYMUS GLAND
Mrs. Priya Gerard

2. THYMUS GLAND
• The thymus gland is a small organ behind the
breastbone that plays an important function
both in the immune system and endocrine
system. Though the thymus begins to atrophy
(decay) during puberty, its effect in "training"
T lymphocytes to fight infections and even
cancer lasts for a lifetime.

3. • The thymus gland lies in the chest, directly
behind the breastbone (sternum), and in front
of the heart in the area between the lungs
called the anterior mediastinum. Sometimes,
however, the thymus gland is found in another
(ectopic) location, such as in the neck, the
thyroid gland, or on the surface of the lungs
(the pleura) near the area where the blood
vessels and bronchi enter the lungs.

4. • It is named the thymus due to its shape being
similar to that of a thyme leaf—pyramid
shaped with two lobes. The two lobes of the
thymus are broken down into lobules. These
lobules have an outer cortex occupied by
immature T lymphocytes, and an inner
medulla occupied by mature T lymphocytes.
• It is considered a lymphoid organ (an organ of
the immune system) similar to the tonsils,
adenoids, and spleen.

5. Cells of the Thymus
• A number of different cell types are present within the thymus gland.
• Epithelial cells: cells that line body surfaces and cavities
• Kulchitsky cells: cells that are the hormone-producing cells of the thymus
or neuroendocrine cells
• Thymocytes: cells that become mature T lymphocytes
• The thymus gland is also home to some macrophages. Macrophages are
known as the "garbage trucks" of the immune system because they eat
foreign matter. Dendritic cells, and a few B lymphocytes (the types of
lymphocytes that produce antibodies) also reside in the thymus.
• Interestingly, the thymus gland also contains some myoid (muscle-like)
cells.

6. CHANGES WITH AGE
• The thymus gland is large in infants, but after
infancy, it grows and reaches its maximum size
during puberty.
• After puberty, the gland shrinks and becomes
largely replaced with fat.
• The gland is very small in elderly people, but can
sometimes atrophy prematurely in response to
severe stress. The term used to describe the
atrophy of the thymus gland with age is "thymic
involution."

7. FUNCTIONS
• The thymus gland is very active from before
birth until puberty, and it functions as both a
lymphatic organ and an endocrine organ (an
organ of the endocrine system that produces
hormones). In order to understand the role
the thymus gland plays in immunity, it's
helpful to first distinguish between T
lymphocytes and B lymphocytes.

8. T Cells vs. B Cells
• T cells (also known as T lymphocytes or thymus
derived lymphocytes) mature in the thymus gland
and play a central role in cell-mediated immunity,
meaning that the cells themselves are active in
fighting off foreign invaders such as bacteria,
viruses, cancer cells, and more.
• In contrast, B lymphocytes are part of the
humoral immune system and produce antibodies
directed at specific invaders.

9. T Cell Training Ground
• As part of the adaptive immune system, the
thymus can be thought of as the training
ground for T lymphocytes. During childhood,
immature T cells (called progenitor cells) that
originate in the bone marrow travel via the
bloodstream to the thymus gland where they
mature and differentiate into specialized T
cells.

10. Types of T Cells
• T cells in the thymus differentiate into three primary
types.
• Cytotoxic T cells. The word cytotoxic means "to kill."
These cells are responsible for directly killing infected
cells.
• Helper T cells. These cells are responsible for both
causing production of antibodies by B cells and
activating other types of T cells to address a foreign
invader.
• Regulatory T cells. These cells function as "police."
They suppress both B cells and other T cells.

11. Positive and Negative Selection
• The immature T cells that leave the bone
marrow enter the thymus in the cortex
(known as the classroom of the thymus).
During "training," these cells are taught to
recognize antigens associated with foreign
cells and matter in a process called positive
selection. Cells are positively selected for
usefulness.

12. Positive and Negative Selection
contd….
• Once the T cells have learned to recognize
specific pathogens, they travel to the medulla
to undergo "negative selection." In the
medulla, the mature T cells are introduced to
the body's own antigens. Since T cells that
would react with the body's antigens could
attack a person's own cells, these cells are
eliminated. T cells are negatively selected for
autoimmunity, and these self-attacking cells
either die or are turned into regulatory cells.

13. Contd……
• Not all T cells make it through this selection
process, and only around 2 percent eventually
make it through positive and negative selection.
• The survivors are then exposed to hormones
produced by the thymus gland to complete their
maturation before being released to do their job
(circulating in the bloodstream or waiting in the
lymph nodes for foreign invaders).

14. Hormone Production
• The thymus gland produces several hormones including:
• thymopoietin and thymulin, which are hormones that assist
in the process where T cells differentiate into different
types
• thymosin, which accentuates the immune response as well
as stimulating pituitary hormones such as growth hormone
• thymic humoral factor, which acts similarly to thymosin, but
increases the immune response to viruses in particular
• The thymus gland may produce small amounts of some
hormones produced in other areas of the body, such as
melatonin and insulin. Cells in the thymus gland (such as
epithelial cells) also have receptors through which other
hormones can regulate its function.

15. ROLESOF MATURED T CELLS
1. Immunity
• T cells are part of the adaptive immune system, in
which each T cell has been trained to recognize a
particular antigen. When exposed to a foreign
cell, cytotoxic T cells lock onto the cell and kill it
with assistance from helper and regulatory T
cells.
• This is also referred to as cell-mediated immunity,
as it involves the use of immune cells to fight
infections.

16. 2. Autoimmunity
• In general, T cells are barricaded in the cortex of the
thymus so they do not become sensitized to the body's
own cells. However, the process of negative selection
in the medulla is used to get rid of cells that
accidentally have become sensitized to "self."
• This function helps prevent the development
of autoimmune disorders, which are medical
conditions in which the body attacks its own tissues
rather than foreign invaders. If the thymus gland is
removed early in life, a person has an increased risk of
developing one of these disorders.

17. 3. Aging
• In recent years it's been determined that aging
isn't simply a process in which the body wears
out, but is actually an active process. In other
words, we are designed to age, and the involution
of the thymus gland may be a form of
programmed aging, with the involution
(beginning around age 60) being the trigger for
the deterioration of the immune system with age

18. Diseases and Disorders
• There are a number of diseases and disorders
that can affect the thymus gland, ranging from
genetic disorders that are evident at birth, to
cancers that are most common in older adults.
These disorders can lead to problems with
immunity and autoimmunity, such as
myasthenia gravis and
hypogammaglobulinemia.

19. Hypoplasia/Aplasia of the Thymus
• The developmental disorder called DiGeorge
syndrome is an uncommon condition marked
by a significant reduction or absence of
thymus function. Caused by a gene mutation,
children with the condition have severe
immunodeficiency and a high risk of
infections, as well as hypoparathyroidism.

20. Thymic Follicular Hyperplasia
• Enlargement (hyperplasia) of lymphoid follicles in
the thymus gland is often seen in autoimmune
diseases such as myasthenia gravis, Graves'
disease, and lupus.
Thymic Cysts
• On their own, thymic cysts are often an incidental
finding, but they can be important in that they
sometimes hide cancer (thymoma or lymphoma).

21. Tumors of the Thymus Gland
• Thymomas are tumors that arise in thymic epithelial cells of
the thymus gland and may be benign (usually harmless) or
malignant (cancerous). They may occur in the usual
location of the thymus gland in the mediastinum, but also
in other regions where the thymus gland is sometimes
located such as the neck, thyroid gland, or on the lungs.
• Other tumors that may occur in the thymus include thymic
lymphomas, germ cell tumors, and carcinoids.
• Symptoms of thymomas may be related to the location of
the tumor in the chest (such as shortness of breath), but
these tumors may also be discovered due to paraneoplastic
syndromes associated with the tumor.

22. There are several of these types of
conditions.
• Myasthenia Gravis (MG). The autoimmune condition myasthenia
gravis occurs in roughly 25 percent of people with thymomas,6 but
may also occur with thymic hyperplasia. MG is an autoimmune
neuromuscular disease caused by problems in communication
between nerves and muscles. It is characterized by profound
weakness of muscles (both in the extremities and respiratory
muscles—this can lead to breathing problems).
• Pure Red Cell Aplasia. This condition is a rare autoimmune disorder
in which T cells are directed against the precursors of red blood
cells, leading to severe anemia. It occurs in roughly 5 percent of
people with thymomas.7
• Hypogammaglobulinemia. Hypogammaglobulinemia (low levels of
antibodies) occurs in roughly 10 percent of people with thymomas