| Noun | 1. | immune system - 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 responsesystem - a group of physiologically or anatomically related organs or parts; "the body has a system of organs for digestion" bone marrow, marrow - the fatty network of connective tissue that fills the cavities of bones thymus gland, thymus - a ductless glandular organ at the base of the neck that produces lymphocytes and aids in producing immunity; atrophies with age RES, reticuloendothelial system - a widely distributed system consisting of all the cells able to ingest bacteria or colloidal particles etc, except for certain white blood cells | |