Definition of immune system
: the bodily system that protects the body from foreign substances, cells, and tissues by producing the immune response and that includes especially the thymus, spleen, lymph nodes, special deposits of lymphoid tissue (as in the gastrointestinal tract and bone marrow), macrophages, lymphocytes including the B cells and T cells, and antibodies
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Merriam-Webster unabridged
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Time Traveler for immune system
The first known use of immune system was circa 1919
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