Bananas with black spots produce TNF (Tumor necrosis factor).
TNF is well known for fighting abnormal cells and having this in mind, bananas that are really ripe, especially those with black spots on its skin are really beneficial for the immunologic system.
During the process of ripening the amount of antioxidants in them drastically increase and it's starch is converted into sugars, which facilitates their digestion a lot.
Putting it all together with its high anti-cancer properties, Japanese scientists recommend the consumption of 1-2 of them a day.