>And we still haven't figured out all of the details of how plans actually work.

You can say that about basically everything we know. There is always another, deeper level, that we don't really know how it works. We just know the results it produces on the level that we do understand.

Lets simplify this greatly.

The immune system is a very complex interplay of just about everything in our lives. We have components that just are, and dynamic components which react to stimuli.

It is very unlikely for two healthy people where one gets sicker than the other to have a truly pinpointable problem, unless they have an undiagnosed biochemical/immunological problem.

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It's plants. They grow in the ground in front of us. We've have discovered Agriculture about 10k years ago.

And we still haven't figured out all of the details of how plans actually work.

It's humbling and amusing to think about how we're always dreaming about space, other worlds, or the unexplored deep sea trenches, but could just as well just spend more time studying the grass we're standing on.

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The immune system is better thought of as being good if it's balanced rather than strong.There's also still a lot we don't know about what makes the immune system stay in balance and react in the best way possible for the organism, but generally maintaning optimal health and homeostasis in the organism is probably the best answer.

So yeah, like you said we know being sedentary, being overweight, having a poor gut microbiome, poor sleep, drug/alcohol intake etc lead to poor health and also an unbalanced and poorer immune system. As most of us know now being obese was one of the most significant risk factors for Covid 19 mortality for example.

We also know that woman of fertile age have a more active immune system as they have to interact with and protect a fetus, meaning they might get sick less often but they also as a group suffer more auto-immune disease as a side effect.

TLDR: Good health and balance in the organism -> optimally balanced immune system.

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Everything that you mentioned plays a role the immune system. Genetics is an important variable. Some people genetically have stronger immune responses that protect them from infection. Some people have genetic variations that cause them to be more prone to autoimmune diseases. There is a scientific journal titled Genes and Immunity that focuses on this topic.

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But not all immune systems are alike or normal, so what makes for those that are less effective, in the sense that the person gets actually sick more often? (Again, other variables like diet and exercise and sleep and age being equalized if possible - looking for info from scientific studies)

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They do, and it is only just becoming understood. Nitrogen fixing bacteria live inside the plant on and in nodes on the roots, where they exchange nutrients for sugars.

There are also free living nitrogen fixing bacteria that associate with plants within the biofilm they secrete to coat their roots. Plants will also secrete other compounds to recruit bacteria and fungi partners who specialize in extracting certain nutrients the plant is deficient in.

But this process occurs on the surfaces, the tips, and intracellularly within the roots, as well as within special chambers within the roots that host symbiotic fungi.

Each plant species has preferences for the bacteria and fungi it associates with. But they definitely live within the plant, in ways we are just learning.

So would people who “don’t get sick” often be more likely to die from certain things where their under reactive immune systems are a problem? And if so, what kinds of things?

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Yes, it is in many ways different than ours but they have bacteria on an in them that helps crowd out more harnful organisms and process nutrients, many famously have very tight connections with fungi in their root systems to ferry nutrients and even signals arouns, things along those lines. They also have common but non lethal parasites just like animals in addition to .ore destructive ones.

The details vary with what kind of plant you're talking about.

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