What is the immune system?
Unlike most of the other body systems, the immune system is scattered throughout the body. The main defence against invaders such as bacteria and viruses are white blood cells called lymphocytes. These blood cells are made and stored in the body’s lymphatic system, which is a network of thin tubes running throughout the body. It contains watery liquid called lymph, which it drains from the tissues and returns to the blood. At intervals along the length of the lymph vessels are small lumps called lymph nodes. Lymphocytes are stored in these lymph nodes. Waves of lymphocytes are released when the body is injured, or when invaders are detected, and the lymphocytes swarm to the damaged area.
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