EGCG

Camellia Sinesis is the tea plant, the plant species whose leaves and leaf buds are used to produce tea. Oolong tea, black tea, white tea, and green tea are all derived from this tea plant that are all processed differently to attain different levels of oxidation. Green tea differentiates from the rest of the other teas because of its level of EGCG (epigallocatechin-3-gallate).
EGCG is one of most powerful polyphenol and it has antioxidant properties.  Antioxidants are involved in the prevention of heart disease, cancer and diabetes.  It helps to stabilize highly reactive molecules known as free radicals that can damage blood vessels and increase the risk of heart disease, or alter the DNA in our genes, promoting cancerous changes.

The process of aging of tea leaves decreases the amount of antioxidants they contain, which seems to explain why green tea is a more powerful disease fighter than other teas.   Human studies show that the antioxidant capacity of the blood does increase about an hour after drinking green tea. The difference among green tea drinkers in population studies may mean that green tea benefits some people more than others.
Eating a mostly plant-based diet, maintaining a healthy weight, keeping physically active and avoiding tobacco are the most effective steps we can take to lower our cancer risk.

Reference:
Tea: The Green Giant
By PsychologyToday.com
URL: Http://health.msn.com/centers/alzheimers/articlepage.aspx?cp-documentid=100125444

The Growing Evidence for Green Tea
By Karen Collins, R.D., American Institute for Cancer Research
URL: http://health.msn.com/dietfitness/articlepage.aspx?cp-documentid=100109111

Green Tea May Give Full Body Protection
From American Institute for Cancer Research
URL: http://health.msn.com/centers/cancer/articlepage.aspx?cp-documentid=100150901>1=9193