Thymus Gland and T Cells - Functions, Myasthenia Gravis and AIDS
Location and Appearance of the Thymus Gland
What is the Thymus Gland?
The thymus gland, or thymus, is a two-lobed structure in the upper chest. It's located behind the sternum (breastbone) and on top of the trachea (windpipe) and heart. The gland is largest in children and starts to decrease in size at puberty. By the time we reach old age it's almost invisible. The thymus is an important part of the immune system in younger people and may affect the life of older people as well.
A type of white blood cell called a T lymphocyte or a T cell matures in the thymus. T cells destroy viruses, bacteria and parasites and fight cancer cells. The thymus gland also makes hormones called thymosins which help the immune system function and have many other jobs In the body.
Problems in the thymus gland or with T cells are involved in several diseases, including myasthenia gravis and AIDS. As our knowledge of the immune system increases, we will hopefully be able to treat diseases such as these more effectively.
Types of Blood Cells
The red bone marrow makes all of our blood cells, including the red blood cells, the white blood cells and the platelets. Red blood cells transport oxygen to where it's needed. They are also known as erythrocytes. Platelets, or thrombocytes, play an essential role in the blood clotting process that heals wounds. White blood cells are also called leukocytes. Their job is to fight infections and harmful materials. Lymphocytes are one type of white blood cell.
Fighting invading microbes and dangerous materials is a complex job. Five types of white blood cells are involved in the task. In order of abundance, these white blood cells are neutrophils, lymphocytes, monocytes, eosinophils and basophils.
B and T Lymphocytes or Cells
Although white blood cells are created in the bone marrow, they may not mature there. B lymphocytes or B cells do mature in the bone marrow, which is why they were given the "B" designation. These cells are involved in the production of proteins called antibodies which help to inactivate invaders. Another type of lymphocyte is the T lymphocyte or T cell, which travels in an immature form from the bone marrow to the thymus gland, where it matures. There are several types of T cells, each with different jobs.
The Thymus Gland and T Cell Maturation
The cell membrane is the outer covering of a cell. As the T cells mature in the thymus gland, they develop receptors on their cell membranes. These receptors can join to molecules known as antigens present on the surfaces of infected or cancerous cells. Once a T cell receptor is joined to an antigen, the T cell causes the death of the cell bearing the antigen either directly or indirectly.
Many different types of receptors are formed on the maturing T cells, but all the receptors on a particular T cell are of the same type. Some T cells develop receptors that can join with molecules on the body’s own cells instead of antigens on dangerous cells. However, most of these lymphocytes are destroyed in the thymus gland. The mature T cells that are ready to protect the body are released into the bloodstream.
The Natural Killer Cell in the Innate Immune System
Types of T Cells
Natural Killer or NK cells destroy a wide variety of dangerous cells. They are part of the innate or non-specific division of the immune system.
Cytotoxic T cells, sometimes called killer T cells, destroy tumour cells and cells infected by viruses. They are also called CD8+ (CD8 positive) cells because they have the CD8 protein on their surface membrane. This protein acts as a receptor. Cytotoxic T cells are part of the adaptive or acquired division of the immune system and attack specific antigens.
Helper T cells don’t kill cells directly but stimulate other types of immune system cells to attack. Helper T cells are also known as CD4+ cells since they have the CD4 protein or receptor on their cell membrane.
Regulatory T cells (also called suppressor T cells) suppress the activity of the immune system once a pathogen (an organism that causes disease) has been destroyed, which helps to reduce the possibility that the immune system will attack the body’s own tissues.
Memory T Cells aren't well understood, but they somehow "remember" a strategy used to destroy an invader. They quickly spring into action and use this strategy again if the invader enters the body in the future.
Activation of Helper T Cells
Some white blood cells act as phagocytes. Phagocytes surround and engulf invaders, a process called phagocytosis. Once certain phagocytes have “eaten” a pathogen such as a virus or bacterium, they travel to a lymph node or to the spleen, which are sites where mature T cells congregate. The phagocytes digest the pathogen and then send a piece of the pathogen’s body to their cell membrane, where it acts as an antigen. A phagocyte, which is now known as an antigen-presenting cell, “presents” its newly-acquired antigen to helper T cells.
A helper T cell with the correct receptor joins to the presented antigen. This begins the process of helper T cell activation. The activated T cell releases proteins called cytokines which trigger B cells, cytotoxic T cells and other white blood cells to become active and destroy the pathogen. Unlike other components of the immune system, T cells destroy cells containing pathogens but can't detect or attack free-floating antigens, such as those on viruses in the bloodstream.
The HIV virus binds to a T cell and then injects its genetic material or RNA into the cell. The RNA instructs the cell to make new virus particles, or virions. The virions then leave the T cell.
AIDS and Helper T Cells
The human immunodeficiency virus (HIV) that causes AIDS infects and kills the helper T cells. This means that the immune system becomes weaker and is less able to fight infections.
Doctors measure the helper T cell level in the blood to help them discover how an HIV infection is progressing. This is referred to as a CD4 count. A normal CD4 count is around 1000 CD4+ cells per cubic millmetre (or microlitre) of blood but may range from 500 to 1500 cells per cubic millimetre. Although the CD4 count varies somewhat according to factors such as time of day or the use of certain medications, in general if the CD4 count is decreasing the HIV infection is increasing. Someone with an HIV infection and a CD4 count of less than 200 CD4+ cells per cubic millimetre of blood is considered to have AIDS.
Treatment of AIDS aims to reduce the level of HIV in an infected person and to raise the CD4 count so that the person’s body can fight other infections.
The Incredible Shrinking Thymus Gland
The thymus gland was once thought to be simply a place in which lymphocytes died and was believed to be an unimportant structure. This view of the thymus changed in 1961 when scientist Jacques Miller discovered that the gland was really an important part of the immune system. In addition, in 1967 he discovered that there are two distinct types of lymphocytes - the B lymphocytes and the T lymphocytes. We know today that the thymus gland is very active, at least in younger people.
The thymus has a pink-grey color in children. As can be seen in the illustration above, it's a large structure in young people. As a person enters puberty, however, the gland begins to decrease in size and is gradually replaced by yellow fatty material. Scientists have found that the shrinkage of the thymus is caused by the production of sex hormones. By the time someone becomes a senior citizen, the thymus gland is so small that it’s hard to find. Most of the body’s T lymphocytes are produced in early life – even before birth - before the thymus gland shrinks.
A disease called myasthenia gravis seems to be linked to abnormal structure and behavior of the thymus. In this disease the thymus gland may stay large even as a person ages.
The Neuromuscular Junction - A Basic Overview
Myasthenia Gravis and the Neuromuscular Junction
Myasthenia gravis is a neuromuscular disorder in which the skeletal muscles controlling the limbs are weakened. The weakness is worse during physical activity. The muscles that control blinking, speaking, swallowing and breathing may also be affected.
Myasthenia gravis is an autoimmune disease - one in which a person's immune system attacks their body. The attack happens at the neuromuscular junction, which is a region containing the end of a nerve cell, the membrane of a muscle cell and the tiny gap between them. When the neuromuscular junction is working normally, the following processes occur
- The end of the nerve cell contains vesicles filled with acetylcholine.
- When a nerve impulse arrives, calcium ions cause the vesicles to move to the membrane of the nerve cell and fuse with it.
- Acetylcholine leaves the vesicles and travels cross the junction.
- The acetylcholine molecules bind to receptors on the muscle cell.
- The union triggers the muscle cell to contract.
In myasthenia gravis, the immune system makes antibodies that block, change or destroy some of the acetylcholine receptors on the membrane of the muscle cell, preventing the acetylcholine from doing its job. As a result, a person experiences muscle weakness.
Myasthenia gravis can't be cured and without treatment can be deadly. Death is rare nowadays, though, since there are several treatments available to improve symptoms. When treated effectively, people with myasthenia gravis can lead a relatively normal life.
Information From The Myasthenia Gravis Association
The Role of the Thymus Gland in Myasthenia Gravis
The link between the thymus gland and myasthenia gravis isn't clear, but there are often changes in the gland in myasthenia gravis patients. Not only is the gland often unusually large in relation to the patient's age, but it may also develop an abnormal structure, including the presence of clumps of immune system cells. The gland may also contain thymomas (tumors of the thymus gland). The thymomas are generally benign, or non-cancerous, but may sometimes become malignant, or cancerous. It's thought that in many cases of myasthenia gravis the thymus gland directs the production of the antibodies that attack the acetylcholine receptors.
A Teenager Talks about MG While Receiving Treatment
More Information About Myasthenia Gravis
The Future - Understanding More About the Thymus Gland
There is some speculation that part of the reason why elderly people experience more incidences of cancer and infections than younger people is because their thymus gland is so small. Some scientists are trying to discover if the thymus gland can be regenerated in elderly lab animals and what effects this will have on their bodies. Another active area of research is the study of thymosins. These hormones have diverse functions and medical applications.
We still have a lot to learn about the human immune system, which is an amazing but extremely complex system. Much more research needs to be done to fully understand the effects of the thymus gland on the body and to discover all the details of the B cell and T cell life cycles.
© 2010 Linda Crampton
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