Stanford immunologist Jorg Goronzy, MD, told me a few years ago that a person’s immune response declines slowly but surely starting at around age 40. "While 90 percent of young adults respond to most vaccines, after age 60 that response rate is down to around 40-45 percent," he said. "With some vaccines, it’s as low as 20 percent."
A shaky vaccine response isn't the only immune-system slip-up. With advancing age, we grow increasingly vulnerable to infection (whether or not we've been vaccinated), autoimmune disease (an immune attack on our own tissues) and cancer (when a once well-behaved cell metamorphoses into a ceaselessly dividing one).
A new study led by Goronzy and published in Proceedings of the National Academy of Sciences, suggests why that may come about. The culprit he and his colleagues have fingered turns out not to be the most likely suspect: the thymus.
This all-important organ's job is to nurture an army of specialized immune cells called T cells. (The "T" is for "Thymus.") T cells are capable of recognizing and mounting an immune response to an unbelievably large number of different molecular shapes, including ones found only on invading pathogens or on our own cells when they morph into incipient tumor cells.
Exactly which feature a given T cell recognizes depends on the structure of a receptor molecule carried in abundance on that T cell's surface. Although each T cell sports just one receptor type, in the aggregate the number of different shapes T-cells recognize is gigantic, due to a high rate of reshuffling and mutation in the genes dictating their receptors' makeup. (Stanford immunologist Mark Davis, PhD, perhaps more than any other single individual, figured out in the early 1980s how this all works.)
T cells don't live forever, and their generation from scratch completely depends on the thymus. Yet by our early teens the organ, situated in front of the lungs at the midpoint of our chest, starts shriveling up and replaced by (sigh - you knew this was coming) fat tissue.
After the thymus melts away, new T-cells come into being only when already-existing ones undergo cell division, for example to compensate for the attrition of their neighbors in one or another immune-system dormitory (such as bone marrow, spleen or a lymph node).
It's been thought that the immune-system's capacity to recognize and mount a response to pathogens (or incipient tumors) fades away because with age-related T-cell loss comes a corresponding erosion of diversity: We just run out of T-cells with the appropriate receptors.
The new study found otherwise. "Our study shows that the diversity of the human T-cell receptor repertoire is much higher than previously assumed, somewhere in the range of one billion different receptor types," Goronzy says. "Any age-associated loss in diversity is trivial." But the study also showed an increasing imbalance, with some subgroups of T cells (characterized by genetically identical receptors) hogging the show and other subgroups becoming vanishingly scarce.
The good news is that the players in an immune response are all still there, even in old age. How to restore that lost balance is the question.
Previously: How to amp up an aging immune response, Age-related drop in immune responsiveness may be reversible and Deja vu: Adults' immune systems "remember" microscopic monsters they've never seen before
Photo by Lars Plougmann