Thymus

What Is Thymus?

The thymus gland is a pinkish grey lymphoid organ that is pyramid shaped and located in the upper anterior part of the chest cavity immediately below the sternum (breastbone) but above the heart.

Despite being a lymphoid organ it also associated with the endocrine system. It is called a thymus gland because its shape resembles that of a thyme leaf. ^1,2

The thymus gland enlarges during childhood until puberty when it reaches its largest size. After puberty the thymus starts shrinking and is replaced by fatty adipose tissue due to the diminished role of the thyroid in adulthood.

Before and during childhood, the thymus produces and matures T-lymphocytes (T Cells), a type of white blood cells that protects the body from foreign disease causing bacteria, viruses and fungi.

Structure

It is divided into two lobes that are further subdivided into smaller parts called lobules. A dense connective tissue capsule covers the thymus and sends fibers into its body for support. Thymus tissue can be categorized into the cortex (outer zone) and medulla (inner zone).

Functions

• The thymus gland produces a hormone called thymosin amongst others that stimulates the development of T cells from the lymphocytes that pass through the thymus. Production of hormones makes the thymus gland and endocrine organ as well as a lymphoid one.
• Positive selection and negative selection are processes carried out in the thymus. The thymus gland receives immature T cells that are produced in the red bone marrow trains them to mature functional T cells that can recognize and attack only foreign cells.
• T cells that attack non-foreign (body) cells die by apoptosis and consumed by macrophages in a process called negative selection.
• Those T cells responding to antigens found in foreign cells survive and mature then migrate to the medulla and enter the bloodstream through the medullary veins in a process called positive selection. Only two percent of immature T cells reach maturity.
• Once T cells are fully mature in the thymus gland, the move to the lymph nodes in the whole body and boost the immune system by fighting disease.
• Particularly they protect the body against re-infectin by the same pathogen by storing a memory of the pathogens antigens and fighting it the next time it invades the body.

Disorders of Thymus

Disorders affecting the thymus can be divided into three major categories.

Congenital disorders

Congenital disorders cause thymus problems from birth.

Severe combined immunodeficiency (SCID) develops in people who carry a mutation in a gene regulating development of T cells; this disrupts the normal development of T Cells and other immune system cells making the child defenseless against infections.

DiGeorge syndrome occurs when a piece of a chromosome along with the genes contained are missing hence causing poor development of the thymus and other immune system organs. Overall, the syndrome causes a weak immune response system that leads to frequent illness. Severity of the disorder ranges from mild to severe.

Autoimmune disorders

They cause the immune system incorrectly perceives the body’s own cells and substances produced by the body’s cells as foreign or harmful hence faultily attacks them.

Myasthenia gravis is the most common autoimmune disease that affects the thymus. It is caused by a large thymus that does not shrink and is abnormally overactive and produces cells which attacks neurotransmitter (acetylcholine) that relays nerve impulses that cause normal muscular contractions.

Thymus cancers

There a two types of cancers that cause cancerous cells to grow on the thymus gland.

Thymoma usually affects people with an autoimmune disease like myasthenia gravis.

Thymic carcinoma (type C thymoma) is more aggressive and difficult to treat compared to thymomas

Symptoms

• DiGeorge syndrome and severe combined immunodeficiency typically cause a weak immunity and susceptibility to frequent illnesses that may take long to heal or neglect to respond to prescribed medication.
• Myasthenia gravis causes the patient to suffer muscle weakness that first manifests in the eyes, slurred speech, difficulty swallowing, and other muscular challenges.
• Thymic cancers usually cause a prolonged cough that does not respond to medication, chest pains and trouble breathing. The early stages don’t present a lot of symptoms.

Diagnosis

Disorders of the thymus gland do not present many symptoms and can go misdiagnosed or undiagnosed for a long time before the condition becomes severe and presents harsh symptoms.

Autoimmune and cancerous disorders of the thymus gland are diagnosed through the combination of the symptoms that point to the disease in question.

Disorders of the thymus gland are usually diagnosed using the following:

• Imaging test like MRI, PET or CT scans
• Chest X-rays
• Biopsy that includes a microscopic examination of the thymus cells.

Congenital diseases are more easily diagnosed as they affect the patient from birth and can be confirmed through genetic testing.

DiGeorge Syndrome is diagnosed by checking infants for signs of low calcium levels due to hypoparathyroidism through blood tests or presence of seizures or ‘jitters’. Symptoms may also mimic those of a heart defect e.g. heart murmurs and arterial blood that is blue or cyanotic due to low oxygen levels.

Treatment

General symptoms that point to a disorder of the thymus gland include:

• Severe combined immunodeficiency is usually treated through bone marrow transplantation and extensive gene therapy.
• DiGeorge’s syndrome is treated depending on its severity. For mild forms the doctor recommends and infusion of extra immune cells.
• Calcium supplementation or replacement of the missing parathyroid hormone also treats this condition. In severe cases the patient may receive a thymus transplant.
• Treatment for myasthenia gravis depends on the extent of the condition. Usually it is treated through prescription of medications in order to suppress the production of abnormal antibodies and improve muscular functions.
• Cancer of the thymus can be treated in a range of ways depending on how long the cancerous cells have had to multiply and spread.
• Surgery is the best way to remove the tumor or thymus gland and other organs that may be infected with the cancerous cells.
• Radiation therapy or chemotherapy could be given before or after surgery to kill the cancer cells using high energy X-rays and chemicals respectively to damage the DNA of the cancerous cells and hence kill them.

There are no home remedies for disorders of the thymus gland and it is advised that one schedule a doctor’s appointment immediately they suspect they have a disorder of the thymus gland.

Reference List

1. Thymus Gland. Retrieved from Britannica – https://britannica.com/science/thymus
2. An Overview of the Thymus. Retrieved from endocrineweb – https://endocrineweb.com/endocrinology/overview-thymus
3. Thymus Gland. Retrieved from InnerBody – https://innerbody.com/image_endoov/lymp04-new
4. Disorders of the Thymus Gland. Retrieved from Livestrong – https://livestrong.com/article/185768-disorders-of-the-thymus-gland/
5. Thymus Cancer. Retrieved from Healthline – https://healthline.com/health/thymus-cancer
6. DiGeorge Syndrome. Retrieved from Immune Deficiency Foundation – https://primaryimmune.org/about-primary-immunodeficiencies/specific-diesease-types/digeorge-syndrome/