The Thymus Gland, T Cells and Disease

The thymus gland is a butterfly-shaped structure in the upper chest, behind the sternum (breastbone) and on top of the trachea (windpipe). It’s largest in children and decreases in size as we age. The thymus is part of our immune system. A type of white blood cell called a T cell matures in the thymus gland. The T cells help destroy viruses, bacteria and other invaders, and fight cancer cells. The thymus gland also makes hormones called thymosins which help the immune system function and have many other jobs.

The red bone marrow makes all of our blood cells, including the red blood cells, the white blood cells and the platelets. The white blood cells fight infections. Lymphocytes are one type of white blood cell. B lymphocytes, also known as B cells, mature in the bone marrow and are involved in the production of proteins called antibodies which help 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.

As the T cells mature in the thymus gland, they develop cell membrane receptors that can join to molecules known as antigens present on the surfaces of virus particles or on the outer coverings of bacteria. Once the T cell receptor is joined to an antigen, the T cell causes the destruction of the particle or cell bearing the antigen.

Many different receptors are formed on the maturing T cells. Each receptor can join to a specific antigen. Some T cells develop receptors that can join with molecules on the body’s own cells instead of antigens on invading cells. However, most of these dangerous T cells are destroyed in the thymus gland. The mature T cells that are ready to protect the body are released into the bloodstream.

Cytotoxic T cells, sometimes called killer T cells, destroy tumor cells and cells infected by viruses. They are also called CD8+ (CD8 positive) cells because they have the CD8 protein on their surface membrane. Helper T cells don’t kill invaders directly but stimulate other types of cells to attack viruses, bacteria and parasites. Helper T cells are also known as CD4+ cells since they have the CD4 protein 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 reduce the possibility that the immune system will attack the body’s own tissues.

Cells called 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 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 start attacking the pathogens.

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 millmeter (or microliter) of blood, but may range from 500 to 1500 cells per cubic millimeter. 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 millimeter 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 thymus has a pink-grey color in children. As a person enters puberty, the thymus gland begins to decrease in size, and is gradually replaced by yellow fatty material. Scientists have found that the shrinkage of the thymus gland 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.

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 thymus 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 which develop in the bone marrow and the T lymphocytes which develop in the thymus gland. We know today that the thymus gland is very active. It contains a large collection of T cells with a huge variety of receptors. The thymus gland has the important job of sorting and selecting those T cells which can recognize and efficiently bind with specific antigens.

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.