Women have two X chromosomes in every cell, but one of them is switched off, like a locked room in a house. With age, the lock on that room works loose, and genes that were meant to stay quiet start to speak up. Scientists spotted this when they compared immune cells from young and old mice, along with data from nearly a thousand people.
The effect is especially striking in CD8 memory cells: these are the cells that remember past infections and attack fast when they meet the same threat again. In old female mice, the number of awakened genes in these cells more than doubled, and the cells themselves began to divide more actively. In humans, the same picture shows up only in women, not in men.
It was already known that women have stronger immunity, but why this leads to different diseases with age remained unclear. Now it is clear that the instability of the silenced X chromosome is concentrated in a specific cell type. This explains why the female immune system becomes both stronger and more dangerous to the body itself as women age. Even today, this knowledge helps explain why infections and autoimmune diseases, in which the body attacks itself, run differently in women, and it points to which cells should be studied to predict age-related risks.
In female mice, the inactive X chromosome in memory CD8 cells more often wakes up in genes located near the very start of the chromosome, as if the lock keeping silence there is weakening.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.