The problem is, if it's too "strong", it starts going after things that it shouldn't, which is why we have autoimmune disorders like:

Addison's disease - where your immune system attacks your adrenal glands, and ruins your cortisol production

Celiac Disease - or gluten allergies (or any allergies for that matter)

Myasthenia Gravis - Where your immune system attacks the bridge between your nerves and muscles, and gives you paralysis when trying to use your muscles repeatedly

Edit: I see that you mightve replied to the wrong reply, and I also just re-explained what PHealthy already did.

I would use the word "Effective/healthy" instead of "strong", and the main ways that people say you can support an effective and healthy immune system are essentially as follows.

Sleep / Hydration / avoiding serious diseases by things like vaccination, as your immune system cant tell the difference between the original thing we vaccinate against, and the vaccine (most of the time, usually natural infection gives a stronger CD8 T-cell response, which is what tells infected cells to lock themselves down and self-destruct without leaking anything out.)