Drinking tea may prime the immune system to fight infections and even cancer, researchers have said.
A cuppa contains infection-beating chemicals
The drink contains particular chemicals which are also present in some bacteria, tumour cells, parasites and fungi.
Because these are present in tea, the body is exposed to them so it can build up a defence against them if it comes up against them as part of a disease.
The chemicals are called alkylamine antigens.
US researchers looked at the effect of the antigens on gamma-delta T cells in the immune system, which act as a first line of defence against infection.
Human gamma-delta T cells were exposed to an alkylamine antigen.
There is a huge body of scientific evidence showing that tea can make a significant contribution to a healthy lifestyle
They were then exposed to bacteria to simulate an infection.
Those cells which had been "primed" fought back against the bacteria, by multiplying up to 10 times and secreting disease-fighting chemicals.
Cells which had not previously exposed to an alkylamine antigen showed no significant response to the simulated infection.
Tea versus coffee
The researchers from Brigham and Women's Hospital and Harvard Medical School in Boston and the University of New Hampshire, Durham, then looked to see if the results were replicated in tea drinkers.
They asked volunteers to drink either five small cups of black tea or coffee daily for up to four weeks.
Green and black teas contain an alkylamine antigen and its precursor, L-theanine, but coffee does not.
After two weeks, gamma-delta T cells from tea drinkers were better able to produce disease-fighting chemicals, but coffee drinkers were not.
The researchers say this suggests that drinking tea can promote a strong immune response, in addition to other known health benefits.
Dr Emma Knight, science information officer at Cancer Research UK told BBC News Online: "The potential health benefits of tea have been discussed for many years, primarily focusing on its anti-oxidant activities.
"This study suggests that the nation's favourite drink might offer yet another, unforeseen, benefit to our health."
She added: "It builds upon previous research, published in 1999, which showed that these molecules could help immune cells grown in the laboratory fight infections.
"The researchers now have early results suggesting that alkylamines in tea might be able to boost our natural immune responses to infections.
"Intriguingly, some cancers also contain alkylamines, opening up the possibility that exposure to these molecules could help our immune systems fight cancer.
"This link is currently very tenuous and further research is needed to address the role of these molecules in cancer."
Bill Gorman, executive director of the Tea Council, said the research was added evidence of the benefits of drinking tea.
He said: "Next to water, tea is the most consumed beverage in the world, and as a result, has attracted an enormous quantity of research over the years.
"There is a huge body of scientific evidence showing that tea can make a significant contribution to a healthy lifestyle, and in particular, in the areas of cardiovascular health and dental hygiene".
But he added: "As with all science, there is some way to go before conclusive proof is established, but the message emerging for the scientific research into tea is very encouraging indeed."
The research is published in the Proceedings of the National Academy of Science.