The Thymus And Pineal Glands

The thymus could be called the incredible shrinking gland. Composed of two lobes held together by connective tissue, the thymus lies in the chest, behind the breastbone and above the heart. It is quite large in the newborn infant and continues to grow throughout childhood. It is at its heaviest just before puberty, weighing in at nearly an ounce and a half. From puberty on, however, it begins to shrink. The thymus is so small in adults that it is difficult to find during dissection.

Fighting foreign invaders

Despite its tiny size, the thymus plays a crucial role in keeping the body in good health. Without it, the immune system could not function properly, and the body's ability to fight infection would be seriously impaired. When a bacteria, virus, or other foreign substance enters, the body must be able to recognize the invader immediately. The immune system has the amazing ability to distinguish cells that belong to the body (self cells) from invading (nonself) cells. Once recognition occurs, the immune system must then destroy these invaders before they multiply and take over. The cells comprising the forces of the immune system are called white blood cells.

These cells engulf foreign invaders, broadcast signals of the invasion, and call in reinforcements. The thymus plays a crucial role in the attack by producing a type of white cell known as the T lymphocyte, or T cell.

Millions of T cells are constantly circulating in the body, ever alert to rally in case of attack. From the time of birth every person has a wide variety of T cells. They help the immune system to combat not only known diseases, such as measles, but also the thousands of new germs that develop each year. T cells are activated through receptors on their cell walls. As they circulate, they come into contact with other white cells that have trapped invading cells.

The T cells respond in several ways. They immediately begin to divide and multiply in a maneuver known as cloning. Four types of T cells result: killer T cells that attack and destroy target cells; memory T cells that launch an immediate attack if the same invader shows up again; helper T cells that call in reinforcements; and suppressor T cells that call off the troops once the battle is under control.

Another class of white cells enter the fray when the helper T cells signal them. These are known as B lymphocytes, or B cells. Some activated B cells become plasma cells. These manufacture all of the antibodies the body produces to fight infection and disease. Antibodies seek out and coat foreign cells, making them easy targets for the many varieties of killer cells in circulation.

The production of T cells and the activation of B cells are largely under the control of hormones manufactured in the thymus. To date, researchers have identified four different products: thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin. There is still much to be learned about these hormones.

Acquired immune deficiency syndrome (AIDS) is an acquired defect of the immune system caused by the human immunodeficiency virus (HIV) that was officially recognized as an epidemic disease by the Centers for Disease Control in 1981. The virus can be passed from one person to another only through the exchange of bodily fluids, mainly blood or semen. A person may be infected with HIV for years before developing symptoms of AIDS. Since 1981, over 700,000 Americans have been diagnosed with AIDS, and over 400,000 have died as a result of AIDS. AIDS devastates the cells of the immune system, particularly T cells. It not only destroys cells but can also inactivate healthy cells from functioning properly.

One area of research on AIDS involves thymosin. Researchers are testing to see if the hormone can help repair a damaged immune system. Noting that administering thymosin helped to reverse immune deficiencies in children born without thymus glands or with malfunctioning organs, they hope that boosting supplies of thymosin will increase the production and action of T cells and B cells in people with AIDS.

The pineal gland

The tiny, deeply buried pineal gland has mystified researchers for centuries. Rene Descartes, one of the great thinkers of the 17th century, credited the pineal gland with being "the seat of the rational soul." Almost all fish, birds, reptiles, and mammals have pineal glands. This seems to have been the case for a very long time. Paleontologists, who study ancient fossils, have found evidence of pineal glands that goes back 500 million years. Although its exact function remains an enigma, the longevity of the pineal body alone indicates that it is clearly of some importance to human survival.

Still, a few things are known with certainty about this gland. The human variety is generally less than one-quarter of an inch long and weighs a fraction of a gram. It is cone shaped, or more exactly, pinecone shaped, which is how it got its name. The pineal resides close to the very center of the brain. At about the time of puberty, it begins to calcify. This produces calcium deposits known as brain sand. Do not worry, though; apparently, this partial calcification has nothing to do with the effective functioning of the brain.

The pineal gland is thought to be the only gland producing the hormone melatonin. In humans, melatonin is secreted throughout the night, and its secretion seems to be directly related to the absence of light. During the day, light enters the eyes. It sends signals through the optic nerve to the spinal cord. These signals are then carried by nerve impulses to the pineal gland. As long as these signals are received, melatonin production is shut down. During the day, the gland prepares for the evening's work by synthesizing amino acids into serotonin. Serotonin converts to melatonin under cover of night.

Melatonin was first discovered in 1958 by researchers at the Yale University School of Medicine. Their work, like much of the work on the pineal gland, was based on research with animals. It appears that melatonin levels directly control the breeding habits of most species. Species with a long gestation period, such as deer and bears, generally conceive in the fall and give birth in the spring. Animals with short gestation periods, such as rabbits and birds, reproduce in the spring and summer. This assures that newborns are given the best possible chance for survival. During the dark, harsh winter, melatonin production is up, and sexual activity is down.

In humans, rising levels of melatonin correlate with increased sleepiness, a craving for carbohydrates, and an inability to concentrate. People who live in climates that have long, dark winters often experience a seasonal depression. Perhaps you have had a case of the "winter blues." If you generally feel depressed, lethargic, withdrawn, and tend to gain weight in the winter, you may qualify as a victim of SAD, or seasonal affective disorder.

This mental health disorder is thought to affect over 30 million people in the United States alone. Most cases occur in the Northeast, the Great Lakes area, and in the states of Washington and Alaska. In some cases, symptoms can become extremely severe. People are unable to cope with even the slightest problems and can barely stay awake at all during the day.

Generally, victims of SAD feel better when spring arrives, and a vacation to a sunnier climate may help to relieve symptoms.

In addition, researchers have begun to experiment with light therapy. It has been found that exposing SAD patients to very bright lights for a few hours a day seems to cure their depression and craving for carbohydrates. Results often occur within a matter of two or three days. The level of light must be at least 10 times as bright as that of a normally brightly lit room.

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