Banana produces the substance named TNF (Tumor Necrosis Factor) that has the ability to fight abnormal cells, but when it ripens it starts developing dark spots or patches on the skin. The more dark the patch is, then its immunity enhancement quality will be higher. The cells that the banana contains may increase the immune system capacity, thus if you eat ripe bananas certain diseases and carcinogenesis may be prevented.
The nutrient content of fruits change a bit when they ripen and when banana ripens it turns yellow while its levels of antioxidants increase and the starch content changes to simple sugars which are easier to digest in full ripe bananas with dark spots on the skin.
There was an experiment conducted by Japanese scientists who found out that this fruit increases the number of blood cells, enhances the immunity of the blood, and finally produces anti-cancer substance TNF. It is recommended to eat 1-2 bananas daily. Moreover, the bananas which have brown spots on the skin are believed to be more effective than the green or fresh ones without those dark patches.
There was a proof that the activity of banana can be compared to that of Lentinan which is a chemical immunostimulant intravenously administered as an anti-cancer agent. In this case, ripe bananas are acting like anti-cancer agents through the process of stimulating the production of white blood cells preventing cell mutation.
Store the bananas into a fridge once they ripe fully in order to minimize further vitamin loss. Those fresh bananas which have brown patches are ripe enough to be eaten momentarily.