That’s a really interesting question, thanks ells8! Immune systems are quite complex things, and can be affected by both your genes and the environment around you. A good example of environmental effects is chickenpox. If you get chickenpox when you’re young, then you’re more likely to develop an immunity, which means you’re less likely to get it when you’re an adult. If you were never exposed to chickenpox (say, because you never came into contact with anyone who had it), then your immune system as an adult will be slightly different to that of someone who did have it. Another example is if you’re on certain types of medicine that can actually suppress your immune system as a side effect, that means your immune system will be slightly different to that of someone who isn’t on that particular medicine. Because there are so many different types of bacteria, viruses, and so on that can affect your immune system, then over time this might play a role in the genetics of your immune system.