“Older people are not as good at reacting to microorganisms they haven’t encountered before,” said physician and immunobiologist Janko Nikolich-Zugich of the University of Arizona College of Medicine. He calls it “the twilight of immunity.”
Our immune systems have two sets of defenses against viruses and other pathogens: a first-line army of cells, called leukocytes, that attack invading microbes within minutes to hours, and a second-line force of precisely targeted antibodies and T cells that surge to the battle front as late as several days after.
With advancing age, the body has fewer T cells, which produce virus-fighting chemicals. By puberty, the thymus is producing tenfold fewer T cells than it did in childhood, Nikolich-Zugich said; by age 40 or 50, there is another tenfold drop.
That leaves the body depleted of T cells that have not yet been programmed to defend against a specific microbe. Fewer such “naïve T cells” means fewer able to be deployed against a never-before-seen microbe.
“We just have fewer soldiers dealing with attackers we’ve never experienced before, like the new coronavirus,” Nikolich-Zugich said. (The body does retain the “memory T cells” that learned to fight attackers in youth, which is why immunization against smallpox and many other viral disease lasts decades.)
