Our immune system is essential for our survival, as our bodies are constantly being exposed to bacteria, viruses, parasites, and other pathogens. Without an immune system, we would quickly lose the war against these pathogens and succumb to these outside invaders. The immune system is made up of billions of individual white blood cells that circulate in our bloodstream and move around in our tissues, patrolling for signs of infection or tissue damage. The body's defenses consist of numerous different types of white blood cells that include lymphocytes, monocytes, and granulocytes. Lymphocytes are in turn further subdivided into T cells, B cells, and NK cells.