Most of the time, nonspecific defenses prevent pathogens from entering the body. Sometimes it can break through and cause illness. This is where the immune system comes into play. The immune system is the body's third line of defense. It is a network of different tissues and white blood cells. The tissues of the immune system are bone marrow, thymus, lymph nodes, spleen, tonsils, and adenoids. The white blood cells of the immune system are called lymphocytes (Postlethwait Hopson, Modern Biology). The immune system's job is to keep "foreign" invaders out of the body or, if they enter, to track them down and kill them. . These foreign invaders are called pathogens, which are tiny organisms that can cause an infection in the body. Pathogens can include bacteria, parasites, and fungi (http://www.niaid.nih.gov/topics/immuneSystem/pages/whatisimmunesystem.aspx). Each part of the immune system plays some role in keeping pathogens out of the body. Bone marrow is the soft material found inside bones. It is used to produce the lymphocytes needed every day in the body. Located above the heart, the thymus is a gland that helps produce a special type of lymphocyte. Lymph nodes are located throughout the body, along the vessels of the lymphatic system. Lymph nodes contain lymphocytes. Lymph nodes collect pathogens from the lymph and expose them to lymphocytes. The largest lymphatic organ in the body is the spleen. The spleen stores healthy blood cells and breaks down aging red blood cells. It also helps produce lymphocytes and other types of red blood cells. Another function of the spleen is to collect pathogens from the blood and attack trapped pathogens. Adenoids are masses of lymphatic tissue found in the nose and throat. The two... at the center of the article... would be able to survive without the help of the immune system. Works Cited “Cells involved in immune responses and antigen recognition.” Microbiology and immunology. Network. December 18, 2011. “Immune Response: MedlinePlus Medical Encyclopedia.” National Library of Medicine - National Institute of Health. Network. December 18, 2011. .Lechtenberg, Richard. "Information book on multiple sclerosis". F. A. Davis Company, 1988. "Modern Biology, (0030565413), Holt Rinehart & Winston." Network. December 18, 2011.Neustaedter, Randall. The immunization decision. North Atlantic Books, 1990. "What is the Immune System." Home page of the National Institute of Allergy and Infectious Diseases. Network. December 18. 2011. .