immune systems

Definition of immune system
National Cancer Institute

immune system

(ih-MYOON SIS-tem)
A complex network of cells, tissues, organs, and the substances they make that helps the body fight infections and other diseases. The immune system includes white blood cells and organs and tissues of the lymph system, such as the thymus, spleen, tonsils, lymph nodes, lymph vessels, and bone marrow.

Source: NCI Dictionary of Cancer Terms