Researchers have found that the addition of milk reduces the benefits of tea in an experiment. The researchers investigated the effect of blood flow in arteries using ultrasound. This measurement of artery functioning is thought to be related to long-term heart disease. This experiment is a prime example of an interaction. That is, milk interacts with tea to attenuate the artery functioning.

(Reuters News has an article describing the research if the above article is unaccessible.)