The Thymus Gland 

Happy Holidays! This month we’re looking at the thymus gland. 

The thymus gland is the body organ that took scientists the longest time to understand. It's only active from when you're born until you become a teenager. Still, it plays a huge role in training your body to fight infections and even cancer for the rest of your life. It's also vital to the body's chemical messaging or endocrine system. 

History and Anatomy 

The thymus gland is usually located behind the breastbone, in front of the heart, and between the lungs. In some people, though, this organ is found in the neck or upper chest. 

While that may seem odd, it's part of the organ's mystery that has baffled scientists for centuries. Researchers only began to understand some of the gland's function about 50 years ago. 

They still aren't sure where it got its name. Some experts say the ancient Greeks, who discovered the organ, named it thymus because its shape looks like the leaves of thyme, a cooking herb. Others say the name came from the Greek word for the soul because the organ is near the heart. 

Either way, the thymus gland is considered an immune system organ. Like your tonsils and adenoids, it helps fight infections.  

Cells of the Thymus 

Unlike your heart or lungs, what the thymus gland does isn't apparent. Its job involves a lot of tiny chemical processes. 

Inside the thymus gland, there are many different cells. These include 

• Epithelial cells line all body surfaces and act as a protective barrier. 
• Kulchitsky cells make hormones, chemical messengers for the thymus and other cells. 
• Thymocytes are cells that become mature T lymphocytes, specialized infection fighters. 
• Dendritic cells are found in the skin and other tissues. They help protect against toxins and other foreign substances. 
• Macrophages are cells that are sometimes called the "garbage trucks" of the immune system. They eat foreign matter and clear away tumors. 
• B lymphocytes are cells that make antibodies, proteins that attack viruses and bacteria. 
• Myoid cells are muscle-like cells. Scientists believe they trigger the autoimmune response in a muscle disorder. 

This list hints at how complex the thymus gland's job is. Its role also changes throughout your life. 

Changes With Age 

The famous Greek philosopher-surgeon, Galen, was the first to notice that the thymus gland changes with age. Galen wrote that it's large in newborn animals and becomes smaller when they're adults. 

The term scientists use to describe this organ shrinking process is called involution. Doctors also know that severe stress can cause the thymus to shrink. In fact, during the early part of the 20th century, researchers didn't believe the thymus was larger in infants. When they did autopsies on babies who had died from conditions like diphtheria, it had shrunk. Your thymus gland reaches its maximum size when you're a teenager. Then, it starts to shrink slowly. By the time you turn 75 years old, your thymus gland turns to fat. 

Function 

From the time you're conceived until you reach puberty, your thymus gland is very active. It serves both the immune and endocrine systems. That's the system that makes hormones, the body's chemical messengers. 

To understand the thymus' immune system role, you need to know the difference between two kinds of white blood cells. They are the T lymphocytes (T-cells) and B lymphocytes (B-cells). These cells are like the immune system's "special ops" forces. 

T Cells vs. B Cells 

T-cells are also known as thymus-derived lymphocytes. They help fight off the body's foreign invaders, such as bacteria, viruses, and toxins. They can also identify and attack cancer cells. 

B lymphocytes, or B-cells, have a different role. They produce proteins called antibodies and use them to destroy specific invaders. 

T Cell Training Ground 

When the immune system's first responders and defenders need extra help, they call in the T-cells. They're made in the bone marrow, the spongy tissue within your bones. When T-cells are young or immature, they travel through the bloodstream and into the thymus gland. 

The thymus has two lobes. One houses a sort of "boot camp" training ground for T-cells. That's where they mature and turn into specialty disease-fighting cells with different jobs. 

Types of T-Cells 

T-cells in the thymus turn into three main immune system disease fighters: 

• Cytotoxic T-cells: These cells are responsible for directly killing infected cells. 
• Helper T-cells: These cells get the B-cells to make antibodies. They also prime the T-cells and get them to attack foreign invaders. 
• Regulatory T-cells: These cells function as "police." They suppress both B-cells and other T-cells if they are mistakenly harming the body. 

Positive and Negative Selection 

The part of the thymus called the cortex is where the T-cell boot camp training is held. Here, young T-cells learn to identify antigens or toxins linked to foreign cells and matter. This process is called "positive selection." 

Once the T-cells recognize specific pathogens, they travel to another part of the thymus gland called the medulla. Here, they get a different kind of training, "negative selection." They're introduced to the body's antigens, so they don't attack and harm them. 

This prevents autoimmune disorders. These are medical conditions where things go wrong, and your cells attack your body tissues and cells instead of foreign invaders. 

Next, the survivors get exposed to hormones produced by the thymus gland to complete their training. Then they are released to do their job. 

Roles of Mature T-Cells 

These highly trained cells circulate in the bloodstream or wait in the lymph nodes until the immune system sounds an alarm. Mature T-cells play some vital roles. 

Immunity 

T-cells are part of the body's adaptive immunity system. That's immunity your body develops after your immune system has been exposed to an infection, vaccine, or foreign substance. 

T-cells are trained to recognize and take out foreign threats that get past the body's first line of defense. When killer cytotoxic T-cells recognize a foreign invader, they lock onto the cell and destroy it with the aid of helper and regulatory T-cells. 

This is what's known as cell-mediated immunity, or using immune cells to fight infections. 

Autoimmunity 

The process of negative selection occurs in the thymus. It is used to get rid of T-cells that have become overly reactive and have bound too strongly to other molecules. The weed-out process clears T-cells that might attack the body's own tissues and cells. This prevents the development of autoimmune disorders. 

Aging 

Scientists used to believe that aging was just the body wearing out. They now realize that aging is an active chemical process. 

Some scientists believe the shrinking of the thymus may be what triggers the aging process. 

As the thymus shrinks, your immunity decreases. That's why older people are more likely to get sick or get diseases like cancer. They're also less likely to respond to vaccines. Studies are now looking at ways to delay the thymus shrinking, boost immunity, and slow the aging process. 

Hormone Production 

The thymus gland produces several hormones, including: 

• Thymopoietin and thymulin: These hormones are involved in the process where T cells get turned into different types of disease fighters. 
• Thymosin: This hormone boosts the immune system's response. Thymosin also stimulates hormones that control growth. 
• Thymic humoral factor: These hormones increase the immune system's response to viruses. 

The thymus gland also makes small amounts of hormones produced in other areas of the body. These include melatonin, which helps you sleep, and insulin, which helps control your blood sugar. 

Associated Conditions 

Many conditions can affect the thymus gland, ranging from genetic disorders to cancers in older adults. These can lead to problems with immunity and autoimmunity. 

Absent or Underdeveloped Thymus 

DiGeorge syndrome is a rare developmental disorder in which the thymus gland is underdeveloped or absent. It is caused by a gene mutation. 

Children with complete DiGeorge syndrome have no thymus gland. This leads to a dangerous deficiency in T-cells. Complete DiGeorge syndrome is treated with a thymus tissue transplant.  

Children with partial DiGeorge syndrome have an underdeveloped thymus and low T-cell count. Partial DiGeorge syndrome often improves as the child gets older. 

DiGeorge syndrome can cause serious immune system problems and a high risk of infections. Children with the condition also have problems with their thyroids or hypoparathyroidism. 

Thymic Follicular Hyperplasia 

With this condition, the thymus becomes swollen and inflamed. This can occur in autoimmune disorders such as: 

• Lupus: In people with this condition, the immune system attacks the joints, skin, kidneys, blood cells, brain, heart, and lungs. 

• Erythematosus: This condition causes the immune system to attack connective tissue. 
• Myasthenia gravis (MG): With MG, the immune system attacks the musculoskeletal system (see more on MG below). 
• Rheumatoid arthritis: In people with this condition, the immune system attacks the joints. 
• Graves' disease: Graves' disease causes the immune system to attack the thyroid gland. 
• Sjogren's syndrome: This condition causes the immune system to attack the cells that make saliva and tears. 

Thymic Cysts 

Cysts are abnormal growths filled with liquid. They're tiny, less than 3 centimeters. They're usually not a problem. 

Thymus gland cysts are often only found when a doctor is treating you for something else. An example is screening for lung cancer. In rare cases, thymic cysts can hide cancer. 

Tumors of the Thymus Gland 

Thymomas are tumors that occur within the thymus gland. They can be harmless or cancerous. They can also occur in the neck, thyroid gland, or lungs. 

Other tumors that may occur in the thymus include thymic lymphomas, germ cell tumors, and carcinoids. Symptoms of thymomas often depend on the location of the cancer. For example, ones in the chest might cause shortness of breath. 

Doctors may discover these tumors because a patient has developed what's known as paraneoplastic syndromes. These rare autoimmune disorders occur when T-cells try to fight off a certain kind of cancer tumor. The cells mistakenly attack parts of the brain, spinal cord, nerves, and muscles. 

There are several of these types of conditions: 

• Myasthenia gravis (MG): This is a condition where your muscles weaken and tire, and you lose your ability to control them. This autoimmune condition occurs in up to 25% of people with thymomas. 
• Pure red cell aplasia: This is a condition where your T-cells attack young red blood cells. This causes severe anemia, a lack of oxygen-carrying red blood cells. It occurs in roughly 5% of people with thymomas. 

• Hypogammaglobulinemia: This is a condition where the B-cells don't make enough antibodies. It occurs in roughly 10% of people with thymomas. 

Thymomas may also cause a condition referred to as thymoma-associated multiorgan autoimmunity. This condition is similar to the rejection seen in some people with organ transplants. In these cases, the tumor produces T-cells that attack a person's body. 

Thymectomy 

Surgeons may remove the thymus gland, a thymectomy, under certain conditions. One reason is a baby is born with a heart problem. The thymus is near the heart and large in babies. So, surgeons have to remove it to operate on a baby's heart. 

Surgeons might also remove the thymus if there's cancer in it or if you are diagnosed with myasthenia gravis. Studies show that roughly 60% of people with myasthenia gravis go into remission when their thymus gland is removed. 

The surgery is usually done between puberty and middle age to avoid the potential consequences of removing the thymus gland too early in life. 

Consequences of Thymus Removal 

The thymus gland is important because it trains the immune system to fight infections. Much of this training occurs before birth. However, studies have linked removing the thymus in infants to an increased risk of infections and autoimmune disorders. 

The children also have an increased risk of thyroid disease, asthma, allergies, and possibly cancer. This is because the T-cells play a vital role in preventing cancer. There is also some evidence that removing the thymus can cause early aging of the immune system. 

Summary 

The thymus gland plays a vital role in training the immune system to protect the body against infections, even cancer. This process begins when you're conceived. The thymus gland reaches its maximum size when you're a teenager. Then it starts to shrink slowly. 

Some scientists believe the natural shrinking of the thymus gland triggers the aging of the immune system. That's why as we age, we tend to get sick more and respond to vaccines less. Researchers are studying ways to slow the thymus shrinking process. 

Your thymus gland is essential to both your immune and endocrine systems. The microchemical processes it controls are very complex. Researchers didn't have the scientific knowledge to understand many of them until recently. 

They're still learning. The dramatic rise in autoimmune disorders has increased scientists' interest in the thymus. Even more intriguing is its role in the aging process. It is very likely we'll learn even more about the thymus gland's functions and proper health in the future. 

Author

Joseph Mathews, MD, FACP, FACE, ECNU, CCD Joseph W. Mathews M.D., a board certified Endocrinologist and Medical Director of Palmetto Endocrinology, was born and raised in South Carolina. He earned his Bachelor of Science in Biology from the College of Charleston, Cum Laude. He then achieved his M.D. at the Medical University of South Carolina where he also completed his residency in Internal Medicine and a Fellowship in Endocrinology, Diabetes, and Metabolism. Dr. Mathews is also a Fellow of both the American College of Endocrinology and the American College of Physicians, holds an Endocrine Certification in Neck Ultrasound (ECNU) and is a Certified Clinical Densitometrist (CCD). He has extensive experience performing ultrasound guided fine needle aspiration biopsies. His practice includes a range of specializations including prescribing and fitting patients with insulin pumps. Dr. Mathews' practice has drawn patients from out of state to benefit from his expertise in thyroid disorders, diabetes, cortisol problems and their Endocrine disorders.