Vitals and Bits Inaugural Entry: The Thymus Gland

I’ve always found it really strange that I’m unable to see and touch vast portions of my own body. Never will I hold my own heart in my hands and explore its chambers with my own eyes. I don’t actually know what my lungs, my liver, my kidneys, my spleen, my stomach or my brain look like. All I can see on myself is a bunch of dead skin cells, some tooth enamel, a couple of eyeballs and a smattering of hair. And that’s kind of a shame, if you think about it. I mean, what if my pancreas has a pleasing texture? What if my liver is the Cindy Crawford of livers? What if one of my adrenal glands is shaped like the Virgin Mary? I’ll never know.

Still, Mother Nature has a good reason for keeping these vital bits of anatomy hidden. And she was nice enough to give us plenty of subjective clues about their existence. I can’t see my stomach, but I can feel desperately contracting in anticipation of the cupcake I’m about to shove down my face. And when I lie on my back, I can see and feel the slight pulsation beneath my sternum where my aorta descends into my abdomen. I enjoy how it feels to take in a lungful of Pecos wilderness air. And we’re all familiar with the squirmy discomfort of a dangerously full bladder.

The list of bodily sensations goes on and on. Yet some organs never really reveal themselves. They hum along happily, almost completely obscured from detection. They exert their effects stealthily, content to hide behind the scenes as other major players get credit for their work.

One such ghostly organ is the humble thymus gland (not the be confused with the thyroid). The fact that I’m not dead is the only reason I know I have one.

The thymus gland is part of your immune system. It sits in the middle of your chest, behind your breastbone and on top of your heart. The two lobes that make up the thymus gland can be joined or separate, and they differ slightly in size and weight.

The thymus gland is one of the only organs that reach its peak size in children. At the time of puberty, the thymus begins to shrink and continues to atrophy throughout life. This is because the thymus is responsible for maturation of part of the immune system, which is a process that peaks in childhood.

Immature immune cells known as thymocytes are produced in the bone marrow and then ferried to the thymus, where they grow and mature into T-lymphocytes, or T-cells (T for Thymus! Duh!). T-cells are white blood cells that fight off infections and other nasty problems. Your body produces several different types of T-cells, but they all play a role in immunity.

The importance of T-cells in maintaining health is made evident by the destruction that HIV wrecks upon the human body. This infamous virus behind the AIDS epidemic destroys up to 2 billion T-cells per day, leaving the victim dangerously vulnerable to infection and other immune problems. HIV/AIDS patients often die of opportunistic infections, or infections that rarely cause problems in healthy people. HIV/AIDS patients succumb to these infections because they have so few T-cells to help fight off pathogens.

So what actually goes on in the thymus gland? And why do T-cells need to mature anyways?

It’s vital that your immune system be able to distinguish “self” from “non-self.” For instance, if your T-cells begin attacking your pancreas, you could end up with type 1 diabetes. If your T-cells start picking on your nerve cells, you might wind up with multiple sclerosis. At the same time, if your immune system fails to recognize invaders like bacteria and viruses, you probably won’t be able to survive past infancy. This happens to some children who are born with severe defects of the immune system.

T-cells learn how to differentiate “self” from “non-self” in the thymus. Once they have completed their education, they can be released into the blood stream or other areas of the body to hunt down the bad guys. But the maturation process is very complicated and riddled with pitfalls. About 98 percent of T-cells do not survive the maturation process! Only 2 percent complete the process and graduate to become mature, competent T-cells. Selection of properly functioning T-cells occurs in the thymus.

In the thymus, immature T-cells are tested with potentially dangerous molecules. If the immature T-cell fails to react to these potentially dangerous molecules, the T-cell won’t receive a vital “survival” signal and will die. Similarly, the immature T-cells are tested with bits of “self.” If the T-cell reacts too strongly to the body’s own molecules, the T-cell will self-destruct. Eliminating T-cells that fail either one of these tests ensures that your immune system responds appropriately to dangerous things like bacterial infections but doesn’t go overboard and start attacking everything in sight.

This process, although extremely effective, isn’t perfect. Sometimes defective T-cells survive the selection process. Sometimes healthy T-cells are killed off. Problems with the selection of healthy T-cells can stem from the T-cells themselves, from the thymus, or from any of the other cells and chemicals involved in the process.

Once the thymus gland stocks the body with a good supply of mature, competent T-cells, it begins to shrink and fewer immature T-cells are processed. Active thymus tissue is gradually replaced with fat. Still, some thymus tissue continues to function throughout life. Mature T-cells can also proliferate long after they leave the thymus.

The two main problems we see with the thymus are various immunodeficiencies and cancer. Thankfully, both are relatively rare.

So today, while you feel your muscles burning at the gym or listen to the tympanic gurgle of your post-dinner intestines, take a moment to remember your silent little thymus gland. It may not be palpably pulsating in your chest like your heart, but it’s every bit as important when it comes to producing the sensation of being alive.