1. IMMUNE SYSTEM<br />The immune system is the body’s defense against infectious organisms and other invaders. With the help of many organs, such as the ones in the drawing, it attacks organisms and substances that invade body systems that causes disease. The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. The cells in this system are white blood cells, or leukocytes. These cells are produced and stored in many locations in the body including the thymus, spleen, and bone marrow. These three organs are called lymphoid organs. There are also clumps of lymphoid tissue throughout the body, or lymph nodes, which house the leukocytes. These blood cells circulate through the body between the organs and nodes by lymphatic vessels and blood vessels. <br />