There are countless components in tea that have helpful action in the body (including some acting as antioxidants). The water needs to be hot enough to leach the components from the tea leaves into the water. Not necessarily boiling water is needed.
Something to keep in mind is that the phytochemicals in tea have a half-life of about 4 hours, which means they start to break down substantially after that. So ... it makes sense to drink some tea in the morning and some in the afternoon and evening to keep a wonderful level of antioxidant phytochemicals in your bloodstream throughout the day.
I don't know what you mean about antioxidants in the diet being something to worry about. Generally getting antioxidants from foods vs. supplements keep the amounts sensible and health-promoting and in a balance with other nutrients also found in those foods, many of which have synergy together.
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