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Thymus is critical to immune system

By TOM VALEO
Published April 25, 2006

Despite its obscurity, your thymus gland plays a crucial role in maintaining your immune system.

However, the thymus shrinks radically during adolescence, and continues to shrink throughout life, leaving your body less able to defend itself against bacteria, viruses, cancer cells and other invaders. By the time you reach your 60s and 70s, your immune system may have a harder time fighting off disease because your thymus is so small it can no longer produce a rich supply of T cells, which help your body fight off foreign invaders.

In addition, this shrinkage paradoxically makes your immune system more aggressive in attacking your own body. That makes you more susceptible to such autoimmune diseases as rheumatoid arthritis, lupus and multiple sclerosis.

The thymus is behind the breast bone, on top of the heart, and extends upward, stopping below the neck. It's very large in infants, and continues to grow with the rest of the body during childhood.

But when sex hormones flood the bloodstream during adolescence, the thymus shrinks, the gland filling with fat and connective tissue. By middle age, your thymus gland may have shrunk by 85 percent, and it probably weighs half as much as it did when you were an infant.

"It's the first organ in the body to age,'' said Phong Le, an immunologist at the Loyola University Medical Center in Chicago. "The ironic thing is, we don't know why the thymus ages. We know a lot about the thymus, but we don't know that.''

The thymus transforms stem cells from the bone marrow into protective T cells with the help of a hormone called thymosin, which simultaneously bolsters the body's immunity against outside invaders and suppresses the immune system's tendency to attack the body itself.

The T cells in the thymus are "naïve'' at first - they are ready to fight any invader. When these naïve cells encounter an invader, however, they develop a "memory'' for it, and will launch a rapid and robust immune response whenever they encounter that invader again, even decades later. That's how vaccinations work - they cause T cells to remember an infectious agent so the immune system will be ready to fight it.

As you age, your thymus produces fewer naïve cells, so you can't resist new invaders as effectively, but your immune system accumulates an extensive repertoire of T cells that have a memory of a previous invader, so you're better protected against threats you've encountered in the past.

That may sound reassuring, but these memory cells give off cytokines, which stimulate inflammation, one of the biggest problems in an aging body. The more memory cells you develop in the course of your life, the more cytokines you will have in your system inducing inflammation.

While inflammation can be extremely helpful in destroying bacteria and viruses, it also is damaging to normal tissues. Much heart disease, for example, seems to be aggravated by inflammation of the artery walls.

Inflammation also can cause a stable deposit in an artery wall to rupture, releasing a plug of plaque into the bloodstream, where it can cause a heart attack or stroke.

Many other disorders, including Alzheimer's disease, irritable bowel syndrome, asthma, arthritis and emphysema, are accompanied by inflammation.

What can you do as you age to preserve what's left of your ever-shrinking thymus?

The thymus is extremely sensitive to sex hormones, which is why it shrinks so radically during adolescence, but it also is sensitive to stress hormones. Minimizing stress won't restore your thymus to its youthful vigor, but it will slow further deterioration.

"Stress hormones can kill thymus cells,'' Dr. Le said. "When the stress is removed, the thymus bounces back, but if you have chronic stress, the thymus goes kaput.

"So if you have a nonstressful lifestyle, one can assume the thymus will function better, because it won't be exposed to high levels of stress hormones.''

Tom Valeo is a freelancer who writes about medical and health issues. Write to him c/o Seniority, the St. Petersburg Times, P.O. Box 1121, St. Petersburg, FL 33731 or send e-mail to features@sptimes.com.

[Last modified April 25, 2006, 08:12:39]