📚 Cells of the Immune System | 免疫系统细胞
The immune system is a complex network of cells and molecules that defends the body against infection while distinguishing self from non-self. At A-level, it is essential to understand the different white blood cells, their origins, and their specialised roles in innate and adaptive immunity.
免疫系统是由细胞和分子组成的复杂网络，它保护身体免受感染，同时区分自身与非自身。在A-level课程中，必须理解不同白细胞的来源及其在先天免疫和适应性免疫中的特殊作用。
1. Haematopoiesis and the Origin of Immune Cells | 造血作用与免疫细胞的起源
All immune cells arise from pluripotent haematopoietic stem cells in the bone marrow. These stem cells divide and differentiate along two main lineages: the myeloid lineage, which produces most phagocytes and granulocytes, and the lymphoid lineage, which produces lymphocytes and natural killer cells.
所有免疫细胞都来源于骨髓中的多能造血干细胞。这些干细胞分裂并沿两条主要谱系分化：髓系产生大多数吞噬细胞和粒细胞，淋巴系产生淋巴细胞和自然杀伤细胞。
This process is controlled by signalling molecules called cytokines, such as interleukins and colony-stimulating factors. A-level candidates should be able to link the common origin of immune cells to their structural and functional differences.
这一过程受称为细胞因子的信号分子控制，例如白细胞介素和集落刺激因子。A-level考生应能将免疫细胞的共同起源与其结构和功能差异联系起来。
2. Neutrophils: Rapid Responders | 中性粒细胞：快速响应者
Neutrophils are the most abundant white blood cells and form the first line of defence during bacterial infection. They are phagocytes with a multi-lobed nucleus and granules containing lysozyme and other digestive enzymes.
中性粒细胞是最丰富的白细胞，在细菌感染时构成第一道防线。它们是吞噬细胞，具有多叶核和含有溶菌酶及其他消化酶的颗粒。
Neutrophils leave the blood by squeezing through capillary walls, a process called diapedesis, and migrate towards chemicals released by pathogens or damaged tissues in a process called chemotaxis. Once they engulf a pathogen, the phagosome fuses with a lysosome to form a phagolysosome, where the pathogen is destroyed.
中性粒细胞通过毛细血管壁挤出血管，这一过程称为血细胞渗出；随后向病原体或受损组织释放的化学物质迁移，这一过程称为趋化作用。一旦它们吞噬病原体，吞噬体与溶酶体融合形成吞噬溶酶体，病原体在此被消灭。
3. Macrophages: Big Eaters and Signallers | 巨噬细胞：大吞噬者与信号传递者
Macrophages are large phagocytic cells that develop from monocytes after they leave the blood and enter tissues. They are found in many organs, where they are given specific names such as Kupffer cells in the liver and alveolar macrophages in the lungs.
巨噬细胞是大型吞噬细胞，由单核细胞离开血液进入组织后发育而来。它们存在于许多器官中，并有特定名称，如肝脏中的库普弗细胞和肺中的肺泡巨噬细胞。
Besides engulfing pathogens, macrophages act as antigen-presenting cells. They process engulfed material and display fragments of antigen on their surface using MHC class II molecules, allowing helper T cells to recognise the antigen and initiate an adaptive immune response.
除了吞噬病原体外，巨噬细胞还充当抗原呈递细胞。它们处理吞入的物质，并利用MHC II类分子将抗原片段展示在细胞表面，使辅助性T细胞能够识别抗原并启动适应性免疫应答。
4. Dendritic Cells: Professional Antigen-Presenting Cells | 树突状细胞：专职抗原呈递细胞
Dendritic cells are the most effective antigen-presenting cells and are named for their long, branch-like cytoplasmic extensions. They are stationed in tissues that contact the external environment, such as the skin and mucosal surfaces, where they capture antigens.
树突状细胞是最有效的抗原呈递细胞，因其长而分枝状的细胞质突起而得名。它们驻留在与外界环境接触的组织中，如皮肤和黏膜表面，在那里捕获抗原。
After capturing an antigen, dendritic cells migrate to lymph nodes, where they present the processed antigen to naive T lymphocytes. This interaction is essential for activating the adaptive immune response, because naive T cells rarely encounter antigen directly in peripheral tissues.
捕获抗原后，树突状细胞迁移至淋巴结，将处理过的抗原呈递给初始T淋巴细胞。这种相互作用对于激活适应性免疫应答至关重要，因为初始T细胞很少在外周组织中直接接触抗原。
5. Lymphocytes: The Adaptive Specialists | 淋巴细胞：适应性免疫专家
Lymphocytes are small, round white blood cells responsible for specific immunity. They account for about 20–40% of white blood cells and include B cells, T cells and natural killer cells. Lymphocytes have receptors that recognise specific antigens.
淋巴细胞是负责特异性免疫的小型圆形白细胞。它们约占白细胞的20–40%，包括B细胞、T细胞和自然杀伤细胞。淋巴细胞具有识别特定抗原的受体。
B cells and T cells are involved in adaptive immunity and have immunological memory, whereas natural killer cells belong to innate immunity and respond more rapidly but without antigen specificity. The different lymphocyte types can be distinguished by surface proteins called cluster of differentiation (CD) markers.
B细胞和T细胞参与适应性免疫并具有免疫记忆，而自然杀伤细胞属于先天免疫，反应更迅速但不具有抗原特异性。不同类型的淋巴细胞可通过称为分化簇（CD）标记的表面蛋白加以区分。
6. B Lymphocytes and Plasma Cells | B淋巴细胞与浆细胞
B lymphocytes mature in the bone marrow. Each B cell carries many copies of a specific B cell receptor, which is a membrane-bound antibody. When a B cell binds its complementary antigen and receives help from a helper T cell, it becomes activated.
B淋巴细胞在骨髓中成熟。每个B细胞携带许多特定B细胞受体的拷贝，该受体是一种膜结合抗体。当B细胞结合其互补抗原并接受辅助性T细胞的帮助时，它就被激活。
Activated B cells divide by mitosis to form clones. Some become plasma cells, which are antibody factories that secrete large quantities of soluble antibodies into the blood and tissue fluid. Others become memory B cells, which persist for years and enable a faster secondary response upon re-infection.
活化的B细胞通过有丝分裂增殖形成克隆。一部分成为浆细胞，浆细胞是抗体工厂，将大量可溶性抗体分泌到血液和组织液中。另一些成为记忆B细胞，它们存活多年，使再次感染时能产生更快的二次应答。
7. T Lymphocytes: Helper, Cytotoxic and Regulatory | T淋巴细胞：辅助性、细胞毒性与调节性
T lymphocytes mature in the thymus gland. There are two main types of T cells involved in cell-mediated immunity: helper T cells (CD4⁺) and cytotoxic T cells (CD8⁺). A third group, regulatory T cells, helps suppress immune responses after an infection is cleared.
T淋巴细胞在胸腺中成熟。参与细胞免疫的T细胞主要有两类：辅助性T细胞（CD4⁺）和细胞毒性T细胞（CD8⁺）。第三类为调节性T细胞，有助于在感染清除后抑制免疫应答。
Helper T cells recognise antigen presented on MHC class II molecules and release cytokines that activate B cells, cytotoxic T cells and macrophages. Cytotoxic T cells recognise antigen on MHC class I molecules and kill infected body cells by releasing perforin and granzymes.
辅助性T细胞识别MHC II类分子上呈递的抗原，并释放细胞因子激活B细胞、细胞毒性T细胞和巨噬细胞。细胞毒性T细胞识别MHC I类分子上的抗原，并通过释放穿孔素和颗粒酶杀死受感染的体细胞。
8. Natural Killer Cells: Innate Lymphocytes | 自然杀伤细胞：固有淋巴细胞
Natural killer (NK) cells are large granular lymphocytes that provide rapid, non-specific defence against virus-infected cells and tumour cells. They do not require prior exposure to an antigen and do not have antigen-specific receptors like B and T cells.
自然杀伤（NK）细胞是大型颗粒淋巴细胞，对病毒感染细胞和肿瘤细胞提供快速、非特异性防御。它们不需要事先接触抗原，也没有像B细胞和T细胞那样的抗原特异性受体。
NK cells recognise abnormal cells by detecting reduced levels of MHC class I molecules on the cell surface. When they identify a target, they release cytotoxic granules containing perforin and granzymes, inducing apoptosis in the target cell.
NK细胞通过检测细胞表面MHC I类分子水平降低来识别异常细胞。当识别目标后，它们释放含有穿孔素和颗粒酶的细胞毒性颗粒，诱导靶细胞凋亡。
9. Granulocytes in Allergy and Defence: Eosinophils, Basophils and Mast Cells | 过敏与防御中的粒细胞：嗜酸性粒细胞、嗜碱性粒细胞和肥大细胞
Eosinophils are white blood cells that defend against multicellular parasites, such as worms, and are also involved in allergic reactions. Their granules contain toxic proteins that can damage parasite membranes, but these proteins can also cause tissue damage in asthma and allergies.
嗜酸性粒细胞是防御多细胞寄生虫（如蠕虫）并参与过敏反应的白细胞。其颗粒含有能破坏寄生虫膜的有毒蛋白质，但这些蛋白质在哮喘和过敏中也可能造成组织损伤。
Basophils circulate in the blood, while mast cells reside in tissues such as the skin and respiratory tract. Both contain granules rich in histamine. When they bind allergens via IgE antibodies on their surface, they degranulate and release histamine, causing inflammation, vasodilation and the symptoms of allergy.
嗜碱性粒细胞在血液中循环，而肥大细胞驻留在皮肤和呼吸道等组织中。两者都含有富含组胺的颗粒。当它们通过表面的IgE抗体结合过敏原时，会脱颗粒并释放组胺，引起炎症、血管扩张和过敏症状。
10. Cell Communication and Immune Memory | 细胞通讯与免疫记忆
Immune cells do not work in isolation; they communicate using small signalling proteins called cytokines. For example, helper T cells secrete interleukins that stimulate B cell division and differentiation, while macrophages release cytokines that attract neutrophils to an infection site.
免疫细胞并非孤立工作，它们利用称为细胞因子的小信号蛋白进行通讯。例如，辅助性T细胞分泌白细胞介素刺激B细胞分裂和分化，而巨噬细胞释放细胞因子吸引中性粒细胞到达感染部位。
The adaptive immune system retains memory cells after an infection is cleared. If the same pathogen enters again, memory B cells and memory T cells respond more rapidly and more strongly, often preventing symptoms. This principle underlies vaccination, in which exposure to a harmless form of an antigen creates protective immune memory.
适应性免疫系统在感染清除后保留记忆细胞。如果同一病原体再次进入，记忆B细胞和记忆T细胞会更快、更强地应答，通常能防止症状出现。这一原理是疫苗接种的基础，即通过接触无害形式的抗原来建立保护性免疫记忆。
11. Summary Table of Major Immune Cells | 主要免疫细胞总结表
Use the table below to compare the main immune cells by their lineage and core function. This is a useful revision tool for structured and multiple-choice questions.
利用下表按谱系和核心功能比较主要免疫细胞。这是结构化试题和选择题的有用复习工具。
| Cell type | 细胞类型 | Lineage | 谱系 | Main function | 主要功能 |
|---|---|---|
| Neutrophil | 中性粒细胞 | Myeloid | 髓系 | Phagocytosis of bacteria; rapid response | 吞噬细菌；快速应答 |
| Macrophage | 巨噬细胞 | Myeloid | 髓系 | Phagocytosis; antigen presentation | 吞噬作用；抗原呈递 |
| Dendritic cell | 树突状细胞 | Myeloid | 髓系 | Antigen presentation to naive T cells | 向初始T细胞呈递抗原 |
| B cell / Plasma cell | B细胞/浆细胞 | Lymphoid | 淋巴系 | Antibody production; humoral immunity | 产生抗体；体液免疫 |
| Helper T cell | 辅助性T细胞 | Lymphoid | 淋巴系 | Activates B cells, cytotoxic T cells and macrophages via cytokines | 通过细胞因子激活B细胞、细胞毒性T细胞和巨噬细胞 |
| Cytotoxic T cell | 细胞毒性T细胞 | Lymphoid | 淋巴系 | Kills virus-infected and abnormal cells | 杀死病毒感染细胞和异常细胞 |
| Natural killer cell | 自然杀伤细胞 | Lymphoid | 淋巴系 | Innate killing of infected and tumour cells | 固有免疫中杀死感染细胞和肿瘤细胞 |
| Eosinophil | 嗜酸性粒细胞 | Myeloid | 髓系 | Defence against parasites; allergic responses | 防御寄生虫；过敏反应 |
| Basophil / Mast cell | 嗜碱性粒细胞/肥大细胞 | Myeloid | 髓系 | Release histamine in allergy and inflammation | 在过敏和炎症中释放组胺 |
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