Free radical-related oxidative damage is believed to be associated with a number of health conditions, including cardio-vascular diseases, cancers, inflammatory conditions (eg infections and arthritis) and diabetes just to name a few. Intense researches have been carried out during the recent years to study some natural resources of anti-oxidants. Green tea is one of them. 

Following is from a report by Wanda C. Reygaert, Published in Biomed Research International (Biomed Res Int. 2018; 2018: 9105261)

• The components in green tea that are the most medically relevant are the polyphenols. The most pertinent polyphenols are the flavonoids; and the most pertinent flavonoids are the catechins. The catechins comprise 80-90% of the flavonoids and around 40% of the water-soluble solids in green tea.
• The four main catechins found in green tea are (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG), and (-)-epigallocatechin-3-gallate (EGCG). The most abundant catechin is EGCG (~60%), and the next most abundant is EGC (~20%), then ECG (~14%), and EC (~6%). 
• The research into the effects of green tea on human health has shown that it can be an important dietary factor in the prevention and treatment of various diseases such as arthritis, cancer, CVD, diabetes and obesity, infections, and in neurologic and oral health.