免疫系统(immune system)
身体细胞、细胞产物、器官及构造，察觉与摧毁外来入侵的细菌、病毒及癌细胞。免疫力是基於系统发动对抗入侵者的防卫能力。要让系统运作正常，必须能够分辨身体自己的物质（自身）和外界来源的物质（非自身）。分辨失效会造成自体免疫疾病，免疫系统对没有害处的物质（如花粉、动物皮屑）产生过大或不适当的反应，会造成过敏。系统主要细胞像是辨别抗原的淋巴球，以相关的附属细胞（如巨噬细胞将外来物质吞掉并杀死）。淋巴球是在骨髓从干细胞产生，T淋巴球（T细胞）移动到胸腺而成熟，B淋巴球（B细胞）则是在骨髓中成熟。成熟的淋巴球进入血流，许多与附属细胞寄宿在身体各种组织之中，包括脾脏、淋巴结、扁桃腺及肠黏膜。在这些器官和组织之内，淋巴球受限於结缔组织的纤细网状系统，四处输送去接触抗原。T细胞与B细胞受到适当的刺激，会在淋巴组织进一步成熟与倍增繁殖。淋巴组织排出的体液（淋巴液）透过淋巴管搬运到血液。淋巴结沿着淋巴管分布，过滤淋巴，找出里面所含的巨噬细胞和淋巴球有无抗原存在。脾脏有类似的功能，抽取血样检查有无抗原。淋巴球可以在淋巴组织、血液与淋巴之间穿梭，是免疫系统机能的重要元素。亦请参阅immunodeficiency、immunology。
English version：
immune system
Cells, cell products, organs, and structures of the body involved in the detection and destruction of foreign invaders, such as bacteria, viruses, and cancer cells. Immunity is based on the system's ability to launch a defense against such invaders. For the system to function properly, it must be able to distinguish between the material of its own body (self) and material that originates outside of it (nonself). Failure to make this distinction can result in autoimmune diseases. An exaggerated or inappropriate response by the immune system to nonharmful substances (e.g., pollen, animal dander) can result in allergies. The system's principal cells include lymphocytes that recognize antigens, and related accessory cells (such as phagocytic macrophages, which engulf and destroy foreign material). Lymphocytes arise in the bone marrow from stem cells with T lymphocytes (T cells) migrating to the thymus to mature and B lymphocytes (B cells) maturing in the bone marrow. Mature lymphocytes enter the bloodstream, and many become lodged, along with accessory cells, in various body tissues, including the spleen, lymph nodes, tonsils, and intestinal lining. Organs or tissues containing such concentrations are termed lymphoid. Within these organs and tissues the lymphocytes are confined within a delicate network of connective tissue that channels them so they come into contact with antigens. T cells and B cells can mature and multiply further in lymphoid tissue when suitably stimulated. Fluid (lymph) draining from lymphoid tissues is conveyed to the blood through lymphatic vessels. Lymph nodes distributed along these vessels filter the lymph, exposing macrophages and lymphocytes contained within to any antigen present. The spleen plays a similar role, sampling the blood for the presence of antigens. The capability of lymphocytes to pass between lymphoid tissue, the blood, and lymph is an important element in the system's functioning. See also immunodeficiency, immunology.