An article published in Harvard Health (July 2018) reviewed the best strategies for improving immune functions:
“Like any fighting force, the immune system army marches on its stomach. Healthy immune system warriors need good, regular nourishment. A form of malnutrition that is surprisingly common even in affluent countries is known as "micronutrient malnutrition." Walk into a store, and you will find bottles of pills that claim to boost the health of your immune system. There is no evidence that they actually bolster immunity. Your first line of defense is to choose a diet high in fruits and vegetables.”
You can read the full article here: https://www.health.harvard.