The immune system is the body's overall defense system against harm-causing invaders from the outside world that are capable of penetrating the body in one way or another. The immune system is not one or two organs that do all the work, but rather a network of parts throughout the body that works with other body systems to fight viruses, diseases, infections, and anything that the immune system knows doesn't belong in the body . Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get Original Essay A pathogen is any organism or virus that can cause disease after entering the body. The first line of defense against pathogens is external barriers such as the skin. The nose has nostril hairs that act as a filter for elements entering the body through the nose. The nose can also fill with mucus that can leave the body to eliminate any pathogens that may enter the nostrils. The eyes have eyelashes that can ward off pathogens when a person blinks. The average person blinks about 16 times per minute. This is usually enough to keep out pathogens floating in the nearby air. According to Campbell Essential Biology with Physiology 4th Edition (2012):External barriers also include chemical defenses in the form of antimicrobial secretions. Sweat, saliva and tears contain enzymes that destroy the cell walls of bacteria. The glands produce oils and acids that make the skin inhospitable to many microbes. Concentrated stomach acid kills most of the bacteria we ingest before they can enter the bloodstream. (p. 518) External barriers constitute the first line of defense. Once a pathogen makes it past the barriers, it is now up to the internal immune system to fight it. This is the second line of defense. Phagocytic cells are white blood cells capable of consuming foreign cells or pathogens, thus killing them. Another type of white blood cell that can kill pathogens or infected cells are NK cells, or natural killer cells. These NK cells will discharge chemicals that kill infected cells. The body has another natural defense, which is defensive proteins. When a virus manages to infect a cell, the cell can release interferon molecules. These interferon molecules bind to nearby healthy cells. The binding of interferon molecules to healthy cells prompts the healthy cell to produce antiviral proteins. Other natural defensive proteins created by our immune system are complement proteins, which directly attack pathogens. (Simon, 2012, p. 519) The lymphatic system is a system of vessels and many lymph nodes packed inside white blood cells. Other organs of the lymphatic system include the tonsils, thymus, and spleen. “The two main functions of the lymphatic system are to return tissue fluid to the circulatory system and to fight infections.” (Simon, 2012, p. 520) The lymphatic system works in collaboration with the circulatory system. For example, when someone cuts a piece of paper from a dollar bill, the dollar bill introduces bacteria into and under the skin. The first line of defense has already been penetrated. The lymphatic system will pick up chemical signals that the skin has been torn, then send white blood cells through the arteries of the circulatory system to the site of the paper cut. The white blood cells will engulf the bacteria and cellular debris, allowing the tissue to heal. If all of the above fails to keep your body healthy, there is also one.