The immune system is composed of cells located mainly in the blood, under healthy conditions. These immune cells travel all over the body through the blood, to help us stay healthy. When an organ is infected, for example the nose when we have a cold, the immune cells enter it to cure it. The cells of the immune system are capable of detecting external invaders, such as viruses and bacteria, and they are also able to detect internal problems, such as cancer cells. Once detected, the immune cells attack the problem cells and fight to destroy them. The immune cells then help to repair the damaged organ.
There are different types of immune cells. Some immune cells are present in the body even before birth, ready and waiting to defend us. These cells are part of what is called the innate immune system, and they are specialized for detecting viruses, bacteria, and any other diseased cell. The cells of the innate immune system can quickly alert the entire immune system of the presence of unwanted visitors. However, these cells are not enough to stop the progression of the disease.

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