immune system

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The integrated body system of organs, tissues, cells, and cell products such as antibodies that differentiates self from nonself and neutralizes potentially pathogenic organisms or substances.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol.) The complex of cells, cellular processes, and substances within and diffused throughout an organism which allow the organism to counteract or destroy noxious foreign substances introduced into the body, destroy infectious agents such as bacteria and viruses, destroy malignant cells, and remove cellular debris, thus protecting the organism against many of the potentially harmful external agents and internal events that could lead to sickness or death. The system has numerous interacting components, including circulating antibodies, antibody-producing cells, white blood cells and lymphokines, lymph tissue and lymph nodes, and stem cells which may differentiate into other types of cell, together with the thymus and spleen. The system is responsible for the phenomenon of immunity{3}. See also immunoglobulin and antibody.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy, immunology The system that differentiates self from non-self and protects the body from foreign substances and pathogenic organisms by producing an immune response. It includes organs such as the thymus, the spleen and lymph nodes; tissue such as bone marrow, and lymphoid tissues such as the tonsils; cells such as lymphocytes including the B cells and T cells, and cell products such as antibodies.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a system (including the thymus and bone marrow and lymphoid tissues) that protects the body from foreign substances and pathogenic organisms by producing the immune response