Our immune system fights off lots of infections which could harm us. T cells are an important part of the immune system. They recognize different infections and protect us from them. Our body makes lots of T cells when we are young, but slows down production after puberty. Nevertheless, the numbers of these cells in our body stays the same even when we are old. We wanted to know how T cell numbers manage to stay so stable. We used data from experiments performed on mice to test a variety of potential ways they might do this, using mathematical models. Our experiments and models show that T cells gradually increase their ability to survive the longer they stay in the blood system.