The catechins, which are antioxidant polyphenol compounds, in green tea have demonstrated benefits for the immune system. A double-blind study of 500 people put an ointment made of either a green tea extract sinecatechin, or a placebo, on anogenital warts for a period of up to 4 months. They were followed for 3 months after treatment.

The results were positive. More than half (57%) of those using the green tea extract had their warts clear completely. Only 34% of those using a placebo did. And a whopping 78% of those who used the sinecatechin ointment experienced at least a 50% improvement in their warts.

Another study that sheds light on the green tea-immune system relationship, was a research project released by the Brigham and Women's Hospital in 2003. It showed that theanine may help the immune system when it was faced with infection, by boosting the activity of the gamma delta T cells that are part of our adaptive and innate immunity.

This study followed 11 coffee drinkers and 10 black tea drinkers who each drank 600ml of their favorite drink per day. At the end of 4 weeks blood samples were taken that showed production of these anti-bacterial proteins were up to 5 times higher in the group that drank tea.

Even though this study was done using black tea, it is still very relevant to green tea, as the active constituent responsible for this effect, theanine, is present in both. (Both black and green tea are actually from the same plant - the differences in the beverages come from differences in fermentation and processing).

Theanine is an interesting biochemical in that it counteracts the stimulatory effect of caffeine on the nervous system. That's why drinking tea - black and green - is calming. It takes about 30 to 40 minutes for the calming effect of theanine in tea to be felt. It produces this calming effect two ways

1. it stimulates the production of alpha brain waves. Alpha brain waves are typically produced in meditation, and create a sense of both alertness and relaxation.

2. l-theanine is involved in the creation by the body of GABA (gamma amino butyric acid). GABA has a relaxing effect via its' influence on the two neurotransmitters, dopamine and serotonin. It increases the release of dopamine, and reduces serotonin levels in the brain. The net effect is an improvement in memory and the ability to learn.

A rather interesting green tea immune system study conducted in-vivo (that is, in a lab rather than on animals or people) looked at the effect of green tea's immune properties on viruses. They added an extract of epigallocatechin gallate (egcg) from green tea to samples of the adenovirus, which is one of the cold and flu viruses. It inhibited the ability of the virus to replicate. Another green tea immune system in-vivo study in South Korea found that EGCG had a similar effect on the influenza virus.

In-vivo studies are preliminary studies in that more work needs to be done to establish the same effect in people. However, other green tea immune system studies support the health benefits of green tea for the immune system.

A study in Egypt found that when people drank green tea whilst on a course of antibiotics, the effectiveness of the antibiotics was enhanced. The study looked at the effect of green tea and antibiotics on 28 different disease-causing micro-organisms that belonged to two classes. It found that in nearly every case, and for all antibiotics tested, green tea improved the effectiveness of the antibiotic and seemed to reduce the drug-resistance of some drug-resistant bacteria. And you didn't even need a high concentration of green tea to achieve these green tea immune system benefits in some instances.

Another study from Slovenia found that EGCG, the green tea antioxidant extract, inhibits bacterial DNA gyrase. There is a rather technical explanation of this on Wikipedia and other sites, but all that's really relevant for most people is that two classes of antibiotics actually target the gyrase of bacteria, as humans don't have it.

For most people, green tea immune system benefits may simply mean you are less likely to catch colds and flu throughout the year. And given that green tea tastes rather nice, there is really no reason not to include it in your diet. For those sensitive to caffeine, white tea, which is green tea picked younger, has all the green tea immune system benefits. Green tea contains less caffeine than regular tea, and certainly less than coffee, but it does contain some.

For example, there are 80 mg of caffeine in a 5oz cup of coffee; 40mg in an 8 oz cup of black tea; 20 mg in an 8oz cup of green tea, and 15mg in an 8oz cup of white tea.

For a high quality, standardized extract of green tea made to European standards, with no harsh solvents, click here.

References:

1. http://www.web-us.com/l-theanine_anxiety_reducer.htm

2. http://www.sciencedaily.com/releases/2003/08/030805072109.htm

3. http://www.ncbi.nlm.nih.gov/pubmed/15946418?dopt=Abstract

4. http://www.umm.edu/altmed/articles/green-tea-000255.htm

5. http://www.sciencedaily.com/releases/2008/03/080330200640.htm

6. http://www.sciencedaily.com/releases/2007/01/070116132946.htm

7. http://www.sciencedaily.com/releases/2007/01/070116132946.htm

8. http://www.sciencedaily.com/releases/1999/06/990623062746.htm

9. http://en.wikipedia.org/wiki/DNA_gyrase