Immune cells and foreign particles enter the lymph nodes via incoming lymphatic vessels or the lymph nodes' tiny blood vessels. All lymphocytes exit lymph nodes through outgoing lymphatic vessels. Once in the bloodstream, they are transported to tissues throughout the body. They patrol everywhere for foreign antigens, then gradually drift back into the lymphatic system, to begin the cycle all over again. The immune system stockpiles a huge arsenal of cells, not only lymphocytes but also cell-devouring phagocytes and their relatives.
Some immune cells take on all comers, while others are trained on highly specific targets.
To work effectively, most immune cells need cooperation of their fellows. Sometimes immune cells communicate by direct physical contact, sometimes by releasing chemical messengers. In order to have room for all the cells needed to match millions of possible enemies, the immune system stores just a few of each kind. When an antigen appears, those few matching cells multiply into a full- scale army. After their job is done, they fade away.