The main plants you can use to strengthen the immune system these days are;

Onion, garlic, echinacea, thyme, mint, ginger, turmeric citrus, elderberry, spinach, calendula, thistles.
Of course, it is a very important factor that these are grown without the use of chemicals.
Just as a person with a strong immune system can survive this process without any treatment, a healthy soil will also overcome all diseases without the need for an intervention.