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Immune system

A collection of cells and proteins that works to protect the body from potentially harmful, infectious micro-organisms (microscopic life-forms), such as bacteria, viruses and fungi. The immune system plays a role in the control of cancer and other diseases, but can become the culprit in allergic reactions and the rejection of transplanted organs, tissues and medical implants. The main components of the immune system are the white blood cells. Cancer of the white blood cells is called leukaemia.