To understand how vaccines work, it is helpful to rst look at how the body ghts illness. When germs, such as bacteria or viruses, invade the body, they attack and multiply. This invasion is called an infection, and the infection is what causes illness. The immune system uses several tools to ght infection. Blood contains red blood cells, for carrying oxygen to tissues and organs, and white or immune cells, for ghting infection. These white cells consist primarily of B-lymphocytes, T-lymphocytes, and macrophages: