The term 'lymphocyte site' is a reference to the lymph nodes, which are small, bean-shaped organs located throughout the body. They are part of the lymphatic system, which helps to fight infection and disease. The four-letter solution to this crossword is 'NODE, ' which is another word for a lymph NODE. Lymph nodes act as filters for the lymphatic system, trapping bacteria, viruses, and other foreign substances. They also produce and store white blood cells, which help to fight infection.