When an implant enters the body, the immune system starts building a scar around it, like a conductor setting the music for an orchestra. But the score changes depending on who is in that orchestra: young or old, male or female.
Scientists tested this in mice. They implanted two types of polymer particles into the thigh muscle, made of polycaprolactone and polyethylene, in young (2 months) and old (18 months) males and females. After six weeks they compared which genes switched on, which cells arrived at the implant, and how thick the scar grew. It turned out that sex changes gene activity more than age does: between males and females of the same age, more than 3000 genes differed, while between young and old of the same sex, fewer than 200 did.
What does this mean for humans? Before an implant goes on the market, it is tested on animals. If only young males are used, it's easy to miss that in females or older individuals the material might cause stronger inflammation or a different type of scar. Now it's clear: trials must include both sexes and different ages. Otherwise, an implant that is safe for some will turn out to be problematic for others.
The difference in gene expression between the sexes at a single age exceeded 3000 genes, whereas between ages within one sex there were fewer than 200 differences. This shows how much more strongly sex influences the transcriptome than age does in this model.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.