The Thymus Gland: T Cells, AIDS, and Myasthenia Gravis
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 lobes of the thymus consist of many smaller lobules, which give the gland a bumpy appearance. The thymus is largest in children and starts to decrease in size at puberty. By the time we reach old age it's almost invisible. It's an important part of the immune system in younger people.
A type of white blood cell called a T lymphocyte or a T cell matures in the thymus. T cell activity causes the destruction of viruses, bacteria, and parasites and helps to fight cancer cells. The thymus gland also makes thymosin and other hormones. These help the immune system to function and have many other jobs in the body.
Problems with T cells or in the thymus gland are involved in several diseases, including AIDS and myasthenia gravis. As our knowledge of the thymus and 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. Platelets are actually fragments of a larger cell.
- 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.
- Lymphocytes are one type of white blood cell. There are two main types of lymphocytes—B cells and T cells.
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
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 (special proteins) on their cell membranes. These receptors will eventually 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 cells bearing the antigen either directly or indirectly.
Many different types of receptors are formed on the maturing T cells. A particular T cell has multiple copies of the same receptor, increasing the likelihood that it will be able to attach to a specific antigen. Some T cells develop receptors that can join with molecules on the body’s own cells instead of antigens on dangerous cells. Most of these lymphocytes are destroyed in the thymus gland, however. The mature T cells that are ready to protect the body are released into the bloodstream.
Some Major Types of T Cells
Cytotoxic T cells are sometimes called killer T cells. They destroy tumour cells and cells infected by viruses. They are also known as 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. This 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.
Other types of T cells exist in addition to the ones described above. The cells are a vital component of our immune system.
Activation of Helper T Cells
Helper T cells are a very important component of the immune system because they trigger other cells to destroy pathogens. Like the other types of T cells, they mature in the thymus. The HIV or human immunodeficiency virus destroys helper T cells.
The cells undergo a process known as activation in order to do their job. White blood cells that act as phagocytes play an essential role in this process. 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 its 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.
- The cytokines trigger other white blood cells to become active and destroy the pathogen when they encounter it.
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 virus that causes AIDS infects and kills 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 active HIV in an infected person and to raise the CD4 count so that the person’s body can fight other infections.
Some scientists have raised the possibility of stimulating the thymus of AIDS patients in order for replacement helper T cells to be made. More research is needed to discover how this process might be accomplished, how effective it would be, and whether it would be safe.
The Shrinking Thymus and Myasthenia Gravis
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 colour 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 or MG seems to be linked to abnormal structure and behaviour of the thymus. In this disease the thymus gland may stay large even as a person ages.
MG 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. This region contains 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 numbers refer to the processes shown in the illustration below.
- The nerve impulse (or action potential) travels along the motor neuron and reaches the axon terminal.
- Calcium ions enter the terminal.
- The calcium ions cause synaptic vesicles in the axon terminal to move to the membrane of the neuron and fuse with it. The vesicles contain molecules of a neurotransmitter called acetylcholine. Acetylcholine leaves the vesicles and travels cross the junction between the neuron and the muscle.
- The acetylcholine molecules bind to receptors on the membrane of the muscle cell.
- The binding of the acetylcholine to the receptor allows sodium ions to enter the muscle cell.
- The sodium ions trigger the production of an action potential, which causes the muscle fibres 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 treatments available to improve symptoms. For example, medications that suppress the production of antibodies by the immune system may be administered. This prevents the attack on the acetylcholine receptors.
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 people with the disease. 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. It's thought that in many cases of myasthenia gravis the thymus gland directs the production of the antibodies that attack the acetylcholine receptors.
In some people with MG, the thymus gland contains thymomas (tumors of the thymus gland). The thymomas are generally benign, or non-cancerous, but may occasionally be malignant, or cancerous.
A thymectomy is an operation in which the thymus gland is removed. The operation is sometimes performed to help people with myasthenia gravis. According to the NIH (National Institutes of Health), removing the gland reduces symptoms of the disease in some people and in some patients even cures the illness. This is true even for people who don't have thymomas in their thymus gland.
Understanding More About the Thymus Gland
Some researchers speculate 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. 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. 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 T cell life cycles.
- Thymus gland facts from the University of Leeds
- A beginner's guide to T cells from Cardiff University
- T cell information from European Bioinformatics Institute
- CD4 count for an HIV infection from the U.S. Department of Veteran Affairs
- Myasthenia gravis information from the NIH (National Institutes of Health)
- Facts about the disease from the Myasthenia Gravis Foundation of America
© 2010 Linda Crampton