Have you seen the “internet rumor” that ripe bananas may have an anti-cancer effect? Well, it looks like there may be truth in the tale: Japanese scientists researching fruits and vegetables and their effects on health have discovered that a substance found in ripe bananas, that has been named Tumor Necrosis Factor alpha (TNF-alpha), increases immune capacity; leading to the conclusion that eating ripe bananas has the potential to help prevent lifestyle-related diseases and carcinogenesis. 
The amount of TNF-alpha in a banana increases significantly with ripening of the banana – and bananas that have reached the stage of having multiple brown spots are considered up to 8x more effective than those without. The more dark patches, the higher it is thought the beneficial effects.
TNF-alpha is considered to act as an anti-cancer agent by significantly stimulating the production of white blood cells (Leukocytes), which help to fight abnormal tumor cells in the body. Researchers determined that bananas act in a similar fashion to Lentinan , a chemical immune stimulant that is intravenously administered as an anti-cancer agent.
The beneficial effects were found to be more pronounced in Sweetio (a highland banana cultivar) than in Cavendish (a very common commercially available banana), though cavendish bananas did have a significant effect. There are around 300 wild and 20 edible strains of banana. 
Among the other fruits and vegetables tested, garlic, ginger, cabbage, eggplant, kiwi, watermelon and grape also revealed marked immunostimulant activity.
Phytochemicals are chemical compounds that occur naturally in plants. The study of phytochemicals is of particular interest in the modern age as various phytochemicals have been found by research scientists to play various roles in strengthening the immune system and defending the body against diseases. Whereas earlier research focused on nutrition, the subject of phytochemistry has become more prominent in recent times.
According to Japanese Scientists, ripe bananas (with dark spots on yellow skin) produce a substance named TNF (Tumor Necrosis Factor) which acts as a anti-cancer agent by stimulating the production of white blood cells, and helps to fight abnormal tumor cells in the body. Researchers determined that bananas act similar to Lentinan , a chemical immune stimulant that is intravenously administered as an anti-cancer agent.
To increase immune function a protect against disease, it is a great idea to eat 1-2 ripe bananas per day. In general, in is considered that eating whole, fresh, organic vegetables and fruits is better for health than phytochemical-based dietary supplements: According to the American Cancer Society, “Available scientific evidence does not support claims that taking phytochemical supplements is as good for long-term health as consuming the fruits, vegetables, beans, and grains from which they are taken.” 
While ripe bananas with yellow skin and brown spots contain more sugar than less-ripe (green-skinned) bananas, it is in the form of natural fructose and therefore much healthier than refined sugar.  The carbohydrates in ripe bananas make them the perfect energy boost after or before a workout.
Make sure to avoid bananas with lesions in the skin or otherwise too ripe to eat, as these may be contaminated with bacteria. 
Note – this article (as with the rest of the info on this website) is not medical advice. We report on findings from scientific studies however these should not be considered as proof or medicinal recommendation.
 “Differences in Biological Response Modifier-like Activities According to the Strain and Maturity of Bananas” – Department of Medical Life Chemistry, Japan, 2008. http://www.jstage.jst.go.jp/article/fstr/15/3/15_3_275/_pdf
 David L. Katz, MD. Nutritional Value of Ripe Bananas. http://www.oprah.com/health/Nutritional-Value-of-Ripe-Bananas