Not only quite tasty, but now new research is showing that
watermelon
has good levels of the amino acid citrulline, which they think
convert in the body to arginine, which is an important
cardio-vascular and immune response booster.
"Nothing says you care like the gift of a small watermelon.
At least that's the custom in China, where the offer of this
red-fleshed cucurbit is considered a fine way to please a
gracious host or ill friend."..more background there
A lot of substances stimulates immunity system, tea, honey, vitamins -- mainly vit. C, herbs (danewort and many others which I can't name in english ;-) ) and various commercial products. Some of them help locally (helping caugh out, anti-inflammatory effect etc.), some directly boost growth of white blood cells.
But it can come with a price. Strenghtened immunity can help to develop various immunity-related diseases, such as alergies or atopic dermatitis or others. Therefore, unnecessary continuous usage of immunity boosters is not recommended.
(IANAD -- I am not a doctor, but involved a little)
Watermelon as Immunity Booster
Not only quite tasty, but now new research is showing that watermelon has good levels of the amino acid citrulline, which they think convert in the body to arginine, which is an important cardio-vascular and immune response booster.
"Nothing says you care like the gift of a small watermelon. At least that's the custom in China, where the offer of this red-fleshed cucurbit is considered a fine way to please a gracious host or ill friend."..more background there
a little more technical, Phytonutrients in fruits studies