Most of the time, nonspecific defenses prevent pathogens from entering the body. Sometimes we can break through and cause illness. This is where the immune system comes in. The immune system is the body's third line of defense. It is a network of several tissues and white blood cells. The tissues of the immune system are the 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 job of the immune system is to prevent "foreign" invaders from entering the body or, if there is one, to seek it out and kill it. . These foreign invaders are called pathogens, which are tiny organisms capable of causing 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 preventing pathogens from entering the body. . Bone marrow is the soft material found inside bones. It is used to produce the lymphocytes necessary for the body on a daily basis. Located above the heart, the thymus is a gland that helps produce a particular type of lymphocytes. 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 destroys aging red blood cells. It also helps in the production of lymphocytes and other types of red blood cells. Another function of the spleen is that it collects pathogens from the blood and attacks the trapped pathogens. Adenoids are masses of lymphatic tissue found in the nose and throat. Both... middle of paper ...... would be able to survive without help from the immune system. Works cited “Cells involved in immune responses and antigen recognition”. Microbiology and immunology. Internet. December 18, 2011. “Immune Response: MedlinePlus Medical Encyclopedia.” National Library of Medicine - National Institutes of Health. Internet. December 18, 2011. .Lechtenberg, Richard. “Multiple sclerosis information book”. FA Davis Company, 1988. “Modern Biology, (0030565413), Holt Rinehart & Winston. » Internet. December 18, 2011. Neustaedter, Randall. The vaccination decision. North Atlantic Books, 1990. “What is the Immune System?” » Home page of the National Institute of Allergy and Infectious Diseases. Internet. December 18. 2011. .