IMMUNE SYSTEM WHAT IS IMMUNE SYSTEM ? Immune system is one of the mechanisms in our body that serves as a protective line of defense against unwanted substances that invade our body. The unwanted substances include bacteria, virus, fungi and toxins . Immune system should be able to distinguish between healthy tissues and these unwanted substances so that it can do its work; get rid of them from our body . Other than unwanted substances, it can also recognize and remove dead and faulty cells . White blood cells (leukocytes) – responsible for protecting your body from infection. They circulate in blood and lymph. The spleen – a fist-sized organ in the upper left side of your abdomen, next to your stomach and behind your left ribs. Mainly act as blood filter. The bone marrow – The soft, spongy tissue that has many blood vessels and is found in the center of most bones. Makes nearly all the components of your blood (red bloo