32 Fighting Infection!-High

Fighting Infection! FOCUS Question How do cells in your body work together to fight disease?

The Battle Inside Our Bodies Our bodies repel invaders from the moment we are born. Our skin has special cells that protect and warn us about threats. Mucous membranes in the mouth, throat, eyes, lungs, and bowels provide a formidable barrier. But viruses, bacteria (see cover), fungi, and parasites sometimes get through. We ingest them in food and water. They enter our body through cuts and wounds. In one day, we might get a million unwanted, microscopic visitors. We cannot block every intruder, no matter how hard we try. Every day is a battle. Yet it mostly goes unnoticed. Credit your immune system for this. This immune system includes the organs, tissues, and cells that defend against unseen threats.

Infection Frontlines Has your doctor ever pressed on your neck when you were sick? Inside your neck are lymph nodes. Lymphatic fluid moves throughout the body to screen out invaders. When your lymph nodes are swollen, your immune system is working, probably fighting infection. Your lymphatic system is home to white blood cells, the helpful frontline warriors of the immune system. The lymphatic system works with the circulatory system to help fight infection. The blood stream carries millions of white blood cells throughout the body. There are many types of white blood cells, each with a special job.

On the Attack! Neutrophils (NEW-troh-fills) are white blood cells that seek out and consume viruses, bacteria, and other foreign cells. The neutrophil is first on the scene at an infection site. T lymphocytes (LIM-foh-sites), or T cells, coordinate the other white blood cells. Helper T cells signal the immune system to combat an infection; killer T cells patrol the body and "kill," or eliminate, infected cells. Both types are important for keeping you healthy. Macrophages (MAK-row-fage-is) are the largest white blood cells. They clean up destroyed cells and debris.

Remembering an Infection B lymphocytes, or B cells, are white blood cells that are the memory of the immune system. B cells identify an invader within the immune system. They target the invader for destruction by making antibodies for that specific infection. Antibodies are proteins that attach to specific invaders. If the infection returns, B cells call in the antibodies from their records. The antibodies do not kill the invaders, but rather put a target on them to mark them for destruction by other cells. T cells then arrive to destroy the targeted foreign invaders. Once an antibody is created, B cells can make new antibodies quickly if the invader returns. Word Wise The term "B cells" begins with a B because these cells grow in bone marrow, a part of the immune system.

Immunity The role of B cells explains how vaccinations prevent disease. A vaccine introduces the body to an invader in a way that doesn't make a person sick. Vaccines are made from dead or weakened versions of an invader. By deliberately exposing the body to the dead or weakened invader, the immune system gets a head start at antibody production. If the real invader ever comes along, B cells are ready to produce antibodies and protect the person from attack. The idea behind vaccines came from Edward Jenner. He knew that people who were exposed to a mild form of smallpox suffered no harm when exposed to the stronger version. Today, we call this immunity. Immunity means that you are protected against a certain disease.