When pollen, chemicals, or even house-dust enter the human body, the immune system can over react and go to great lengths to expel them. This is what we call an allergic reaction. More technically, it's when the body produces antibodies triggered by external allergens which bond causing new substances, called chemical mediators, such as histamine. Once mediators are released from mast cells, they cause runny noses, sneezing and itchy eyes.

Hay fever is induced by the chemical mediator histamine. There are various types of anti-allergy medicines that suppress histamine called anti-histamines. Tea catechins have also been confirmed to have the ability to suppress the release of the chemical mediator histamine. In particular, methylated catechin derivatives are effective anti-histamines and are expected to be effective in suppressing allergic reactions but more research needs to be done.