This is interesting... I couldn't find anything at the Snopes (urban legends) web site to refute the info here... but don't know that it is accurate either... still, interesting to take note of   ;)

Health aspects; Interesting to know BANANAS with dark patches on yellow skin - for your info The fully ripe banana produces a substance called TNF which has the ability to combat abnormal cells. So don't be surprised if the shop goes out of stock for bananas. As the banana ripens, it developes dark spots or patches on the skin.. The more dark patches it has, the higher will be its immunity enhancement quality . Hence the Japanese love bananas for a good reason. According to a Japanese scientific research, banana contains TNF which has anti-cancer properties. The degree of anti-cancer effect corresponds to the degree of ripening of the fruit, ie the riper the banana, the better the anti-cancer quality.. In an experiment carried out by a professor on animals in Tokyo comparing the various health benefits of different fruits,using banana, grape, apple, water melon, pineapple, pear and persimmon, it was found that banana gave the best results. It increased the number of white blood cells, enhanced the immunity of the body and produced anti-cancer substance TNF. The recommendation is to eat 1 to 2 banana a day to increase your body immunity to diseases like cold, flu and others. According to the Japanese professor, yellow skin bananas with dark spots on it are 8 times more effective in enhancing the property of white blood cells than the green skin version  .