什麼是免疫系統

身體細胞、細胞產物、器官及構造，察覺與摧毀外來入侵的細菌、病毒及癌細胞。免疫力是基於系統發動對抗入侵者的防衛能力。要讓系統運作正常，必須能夠分辨身體自己的物質（自身）和外界來源的物質（非自身）。分辨失效會造成自體免疫疾病，免疫系統對沒有害處的物質（如花粉、動物皮屑）產生過大或不適當的反應，會造成過敏。系統主要細胞像是辨別抗原的淋巴球，以相關的附屬細胞（如巨噬細胞將外來物質吞掉並殺死）。淋巴球是在骨髓從幹細胞產生，T淋巴球（T細胞）移動到胸腺而成熟，B淋巴球（B細胞）則是在骨髓中成熟。成熟的淋巴球進入血流，許多與附屬細胞寄宿在身體各種組織之中，包括脾臟、淋巴結、扁桃腺及腸黏膜。在這些器官和組織之內，淋巴球受限於結締組織的纖細網狀系統，四處輸送去接觸抗原。T細胞與B細胞受到適當的刺激，會在淋巴組織進一步成熟與倍增繁殖。淋巴組織排出的體液（淋巴液）透過淋巴管搬運到血液。淋巴結沿着淋巴管分佈，過濾淋巴，找出裏面所含的巨噬細胞和淋巴球有無抗原存在。脾臟有類似的功能，抽取血樣檢查有無抗原。淋巴球可以在淋巴組織、血液與淋巴之間穿梭，是免疫系統機能的重要元素。亦請參閱immunodeficiency、immunology。

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.