The secret weapon of your immune system is the ability to distinguish between the body's own cells ("self") and foreign cells ("non-self"). The key players in the immune system are lymphocytes: a variety of white blood cells which are formed in bone marrow and the thymus, congregate in the lymph nodes and other organs, and travel throughout the body in lymph fluid or the bloodstream.
Organs and tissues of the immune system dot the body and are connected by lymph and blood vessels to form a web of protection against infection.
Like soldier bees defending the hive, the detection of "non-self" cells stimulates a burst of lymphocyte activity, swarming to the infection site, reproducing targeted assassins rapidly, producing stockpiles of killer chemicals, and more.
The "warrior classes" of lymphocytes include: