Theory suggests thymus plays role in severity of COVID-19
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Dear Doctor: What’s the connection between the novel coronavirus and the thymus gland? A friend of ours who is a doctor says it’s probably what keeps young kids from getting so sick. I’ve never even heard of the thymus. What does it have to do with coronavirus?
Dear Reader: From the earliest days of the novel coronavirus pandemic, the data revealed a puzzling disparity. Older adults were at increased risk of grave illness when infected with the virus, but children seemed to have a certain level of protection. And while it has since become clear that children can indeed become seriously ill if they become infected, they do so at far lower rates than adults. The reasons for this are still being investigated, but some researchers have recently suggested the role of the thymus gland as a possible factor.
If you place your finger at the notch at the top of your breast bone and draw a vertical line downward a few inches, you’ve traced the location of your thymus. It’s made up of two roughly triangular lobes, which sit behind the breastbone and between the lungs. The thymus has several functions, but perhaps its most important role is to help produce the cells that will become T-lymphocytes, or T-cells. (The “T” stands for thymus-derived.) These are white blood cells that protect the body from bacteria, fungi, viruses and other pathogens.
T-cells, which are the ninjas of the immune system, start out in the bone marrow as stem cells. The immature stem cells exit the marrow, move through the blood and enter a specific region of the thymus. There, they undergo a complex process that teaches them how to recognize a wide range of potentially dangerous and deadly invaders. As T-cells, their job is to circulate throughout the body and, when they encounter the molecular signature of the pathogen they’ve been trained to recognize, to attack. T-cells also activate other immune cells, produce proteins known as cytokines and have a role in regulating immune response.
The thymus is unique in that it reaches maturity in utero and is at its largest and most active in children. Starting at puberty, it gradually becomes less active, and the glandular tissue begins to shrink. This continues throughout a person’s life. By the time someone has reached their mid-60s, the thymus is largely inactive. By their mid-70s, the gland has been mostly replaced with fat. This decrease in thymus function is believed to be one of the reasons that, in their later years, older adults become more susceptible to disease and infection.
Emerging research into COVID-19 has shown a marked decrease in the number of T-cells in some gravely ill patients. Scientists are now asking whether age-related thymus decline, which means T-cells aren’t quickly replaced, may play a role in the severity of illness seen in older adults. The flip side of this is whether, due to their robust production of T-cells, children’s immune systems are able to stay one step ahead of the novel coronavirus. It’s only a working theory, but it shows promise, and research into how this may affect and inform treatment continues.
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Eve Glazier, M.D., MBA, is an internist and associate professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and assistant professor of medicine
at UCLA Health.