📚 Cells of the Immune System | 免疫系统的细胞
The immune system is a complex network of cells, tissues and molecules that defends the body against infection while distinguishing self from non-self. At the heart of this defence are various types of white blood cells, or leukocytes, each with specialised roles in identifying, targeting and eliminating pathogens. In A-level Biology, understanding how these cells develop, interact and contribute to specific and non-specific immunity is fundamental.
免疫系统是一个由细胞、组织和分子构成的复杂网络，它在抵御感染的同时区分“自身”与“非自身”。这一防御体系的核心是多种类型的白细胞，每种细胞在识别、靶向和清除病原体方面各司其职。在A Level生物课程中，理解这些细胞如何发育、相互作用以及参与特异性与非特异性免疫至关重要。
1. Haematopoiesis and the Origin of Immune Cells | 造血与免疫细胞的起源
All immune cells arise from pluripotent haematopoietic stem cells (HSCs) in the bone marrow, a process called haematopoiesis. These stem cells can self-renew and differentiate into two major lineages: the myeloid lineage and the lymphoid lineage, giving rise to the entire repertoire of blood cells.
所有免疫细胞都源自骨髓中的多能造血干细胞，这一过程称为造血。这些干细胞既能自我更新，又可分为两大谱系：髓系和淋系，从而产生全套的血细胞。
The myeloid lineage produces cells involved in innate immunity, such as neutrophils, monocytes (which mature into macrophages), eosinophils, basophils and dendritic cells. Platelets and red blood cells also originate here but are not directly part of the immune cell family.
髓系产生的细胞参与先天性免疫，例如中性粒细胞、单核细胞（成熟后成为巨噬细胞）、嗜酸性粒细胞、嗜碱性粒细胞和树突状细胞。血小板和红细胞也来源于此，但它们并不直接属于免疫细胞家族。
The lymphoid lineage gives rise to lymphocytes: B cells, T cells and natural killer (NK) cells. Lymphocytes are the cornerstone of adaptive immunity and provide highly specific, long-lasting protection.
淋系则产生淋巴细胞：B细胞、T细胞和自然杀伤（NK）细胞。淋巴细胞是适应性免疫的基石，提供高度特异且持久的保护。
2. Neutrophils: Rapid Responders | 中性粒细胞：快速反应者
Neutrophils are the most abundant type of white blood cell, accounting for about 50–70% of all leukocytes. They are short-lived phagocytes that patrol the blood and are recruited rapidly to sites of infection by chemotactic signals released from damaged tissues and resident immune cells.
中性粒细胞是数量最多的白细胞，约占白细胞总数的50–70%。这些寿命较短的吞噬细胞在血液中巡逻，并受损伤组织及固有免疫细胞释放的趋化信号引导，迅速被招募到感染部位。
Once at the site, neutrophils engulf and destroy bacteria through phagocytosis. Their cytoplasm is packed with granules containing lysozyme, defensins and reactive oxygen species that kill ingested microorganisms. Neutrophils also release extracellular traps (NETs) composed of DNA and antimicrobial proteins to ensnare and neutralise pathogens.
到达感染部位后，中性粒细胞通过吞噬作用吞食并消灭细菌。其胞质中充满含溶菌酶、防御素和活性氧的颗粒，这些物质能杀死摄入的微生物。中性粒细胞还能释放由DNA和抗菌蛋白组成的中性粒细胞胞外陷阱（NETs），以捕获并灭活病原体。
Because they are non-specific in their action and cannot distinguish between different pathogens, neutrophils form a crucial part of the innate immune response. Their short lifespan means they die soon after phagocytosis, forming pus.
由于中性粒细胞的作用是非特异性的，无法区分不同的病原体，因此它们是先天性免疫反应的重要组成部分。它们寿命短，在吞噬细菌后很快死亡，从而形成脓液。
3. Macrophages: Big Eaters and Presenters | 巨噬细胞：大吞噬者与呈递者
Macrophages are large phagocytic cells that differentiate from monocytes after leaving the bloodstream and entering tissues. They reside in nearly all tissues and play a dual role: as powerful innate effectors and as antigen-presenting cells (APCs) that bridge innate and adaptive immunity.
巨噬细胞是大型的吞噬细胞，由血液中的单核细胞离开循环进入组织后分化而成。它们几乎定居于所有组织，并扮演双重角色：既是有力的先天免疫效应细胞，又是连接先天和适应性免疫的抗原呈递细胞。
Through phagocytosis, macrophages engulf bacteria, viruses and cellular debris, using a similar arsenal of hydrolytic enzymes and reactive oxygen species as neutrophils. However, unlike neutrophils, macrophages do not die immediately after engulfment; they can survive for months and continue their surveillance.
通过吞噬作用，巨噬细胞吞噬细菌、病毒和细胞碎片，利用与中性粒细胞类似的水解酶和活性氧“武器库”。但与中性粒细胞不同的是，巨噬细胞在吞噬后并不立即死亡，它们可以存活数月并持续进行免疫监视。
Critically, after digesting a pathogen, macrophages process its protein antigens and load peptide fragments onto major histocompatibility complex class II (MHC II) molecules. These MHC II–peptide complexes are displayed on the cell surface, where they can be recognised by helper T cells (CD4⁺). This antigen presentation is essential for activating the adaptive immune system.
关键的是，巨噬细胞消化病原体后会处理其蛋白质抗原，并将肽片段装载于主要组织相容性复合体II类（MHC II）分子上。这些MHC II-肽复合物展示在细胞表面，可被辅助T细胞（CD4⁺）识别。这种抗原呈递对于激活适应性免疫系统至关重要。
4. Overview of Lymphocytes | 淋巴细胞概述
Lymphocytes are small, round white blood cells that are responsible for adaptive immunity. Despite their uniform appearance under the light microscope, they are functionally diverse. The three main types are B lymphocytes (B cells), T lymphocytes (T cells) and natural killer (NK) cells.
淋巴细胞是体积较小、呈圆形的白细胞，负责适应性免疫。尽管它们在光学显微镜下外观相似，功能上却高度多样。主要有三类：B淋巴细胞（B细胞）、T淋巴细胞（T细胞）和自然杀伤（NK）细胞。
B cells and T cells develop initially in the bone marrow, but T cells must then mature in the thymus gland. Each lymphocyte expresses a unique antigen receptor on its surface: the B-cell receptor (BCR) on B cells and the T-cell receptor (TCR) on T cells. The immense diversity of these receptors is generated by gene rearrangement during lymphocyte development, enabling the immune system to recognise millions of different antigens.
B细胞和T细胞最初都在骨髓中发育，但T细胞必须在胸腺中进一步成熟。每个淋巴细胞的表面都表达独特的抗原受体：B细胞表面有B细胞受体（BCR），T细胞表面有T细胞受体（TCR）。这些受体的巨大多样性是通过淋巴细胞发育过程中的基因重排产生的，使得免疫系统能够识别数以百万计的不同抗原。
Lymphocytes circulate between the blood, lymph and secondary lymphoid organs such as lymph nodes and the spleen, patrolling for their specific antigen. Upon encountering a matching antigen and receiving appropriate co-stimulatory signals, a lymphocyte becomes activated, proliferates and differentiates into effector cells and memory cells.
淋巴细胞在血液、淋巴以及淋巴结和脾脏等次级淋巴器官之间循环，巡查与之匹配的特异性抗原。一旦遇到相应的抗原并接到适当的共刺激信号，淋巴细胞便被激活，继而增殖并分化为效应细胞和记忆细胞。
5. B Lymphocytes and Humoral Immunity | B淋巴细胞与体液免疫
B cells are the central players in humoral immunity, which targets extracellular pathogens and their toxins present in body fluids ('humours'). Each B cell displays thousands of identical B-cell receptors, essentially membrane-bound antibodies, specific to one particular epitope of an antigen.
B细胞是体液免疫的核心参与者，体液免疫针对的是体液中存在的胞外病原体及其毒素。每个B细胞表面展示着数千个相同的B细胞受体，本质上就是膜结合型抗体，专门识别某一抗原的特定表位。
When a naive B cell encounters its complementary antigen and receives help from an activated helper T cell – a process known as T-dependent activation – it becomes activated. The B cell undergoes clonal expansion, producing a large number of identical copies. Most of these differentiate into plasma cells, which are antibody factories secreting huge quantities of the same antibody (soluble BCR) into the blood and lymph.
当初始B细胞遇到其互补抗原并接受活化辅助T细胞的帮助（即T细胞依赖性激活）后，该B细胞便被激活。B细胞经历克隆扩增，产生大量相同的拷贝。其中大多数分化为浆细胞，浆细胞如同抗体工厂，向血液和淋巴液分泌大量相同的抗体（即可溶性BCR）。
Antibodies function by neutralising pathogens and toxins, opsonising microbes to enhance phagocytosis, and activating the complement system. A small proportion of activated B cells become long-lived memory B cells, which do not secrete antibodies but remain in the body primed for a faster and stronger response upon re-exposure to the same antigen.
抗体的功能包括中和病原体和毒素、调理微生物以增强吞噬作用以及激活补体系统。一小部分活化的B细胞成为长寿命的记忆B细胞，它们不分泌抗体，而是留在体内，以便再次遇到相同抗原时能更迅速、更强烈地作出反应。
6. Helper T Cells: Master Regulators | 辅助T细胞：主调控者
Helper T cells (Th cells), expressing the surface marker CD4, are the orchestrators of adaptive immunity. They do not directly kill pathogens; instead, they regulate the activity of other immune cells through cytokine secretion and direct cell–cell contact.
辅助T细胞（Th细胞）表达表面标志物CD4，是适应性免疫的“指挥家”。它们不直接杀死病原体，而是通过分泌细胞因子及直接的细胞间接触来调节其他免疫细胞的活动。
Activation of a naive CD4⁺ T cell requires two signals. Signal 1 is the engagement of its TCR with an antigenic peptide presented on MHC II molecules of an antigen-presenting cell such as a dendritic cell, macrophage or B cell. Signal 2 is a co-stimulatory signal, typically the binding of CD28 on the T cell to B7 molecules on the APC. This dual-signal requirement prevents inappropriate activation and ensures tolerance to self-antigens.
初始CD4⁺ T细胞的活化需要两个信号。信号1是TCR与抗原呈递细胞（如树突状细胞、巨噬细胞或B细胞）表面MHC II分子所呈递的抗原肽结合。信号2是共刺激信号，通常是T细胞上的CD28与APC上的B7分子结合。这种双信号要求可防止不恰当激活，并确保对自身抗原的耐受。
Once activated, helper T cells proliferate and differentiate into various effector subsets, such as Th1 cells that enhance macrophage killing of intracellular pathogens, and Th2 cells that promote B cell activation and antibody class switching. They also release cytokines like interleukin-2 (IL-2) that stimulate the proliferation of both T cells and B cells, amplifying the overall immune response.
一旦被激活，辅助T细胞增殖并分化为不同的效应亚群，例如Th1细胞能增强巨噬细胞对胞内病原体的杀灭能力，Th2细胞则促进B细胞活化和抗体类别转换。它们还释放白细胞介素-2（IL-2）等细胞因子，刺激T细胞和B细胞增殖，从而放大整体的免疫反应。
7. Cytotoxic T Cells and Cell-Mediated Immunity | 细胞毒性T细胞与细胞介导免疫
Cytotoxic T lymphocytes (CTLs), identified by the surface protein CD8, are the primary effector cells of cell-mediated immunity. Their job is to identify and destroy host cells that are infected by intracellular pathogens, such as viruses, or that have become cancerous.
细胞毒性T淋巴细胞（CTLs）通过表面蛋白CD8进行鉴定，是细胞介导免疫的主要效应细胞。它们的任务是识别并摧毁被胞内病原体（如病毒）感染的宿主细胞，或已发生癌变的细胞。
Unlike helper T cells, CD8⁺ T cells recognise antigenic peptides displayed on MHC class I molecules, which are found on the surface of all nucleated cells. As almost any body cell can be infected, this widespread expression of MHC I enables CTLs to survey the entire body for intracellular threats.
与辅助T细胞不同，CD8⁺ T细胞识别的是展示于MHC I类分子上的抗原肽，而MHC I类分子存在于所有有核细胞的表面。由于人体内几乎所有细胞都可能被感染，MHC I分子的广泛表达使得CTL能够监视全身的胞内威胁。
Upon specific recognition of a peptide–MHC I complex, and after receiving activation signals from helper T cells, a naive CD8⁺ T cell is activated, undergoes clonal expansion and differentiates into effector CTLs. These CTLs kill target cells by releasing perforin, which creates pores in the target cell membrane, and granzymes, which enter through these pores and induce apoptosis. CTLs can also trigger apoptosis through Fas–Fas ligand interactions.
一旦特异性识别肽–MHC I复合物，并接收到辅助T细胞的激活信号后，初始CD8⁺ T细胞便被激活，进行克隆扩增并分化为效应CTL。这些CTL通过释放穿孔素在靶细胞膜上形成孔洞，并释放颗粒酶通过这些孔洞进入靶细胞诱导凋亡，从而杀死靶细胞。CTL还可通过Fas–Fas配体相互作用引发凋亡。
8. Memory Cells and Immunological Memory | 记忆细胞与免疫记忆
A hallmark of adaptive immunity is immunological memory – the ability to respond faster and more vigorously upon re-encountering a pathogen. This memory is maintained by long-lived memory B cells and memory T cells that are generated during the primary immune response.
适应性免疫的一个标志是免疫记忆，即再次遇到同一病原体时能更快、更强烈地进行应答的能力。这一记忆由在初次免疫应答中产生的长寿命记忆B细胞和记忆T细胞维持。
Memory B cells do not actively secrete antibodies but remain in a quiescent state in secondary lymphoid tissues. When the same antigen enters the body again, these memory B cells are rapidly reactivated, proliferate and differentiate into plasma cells that produce large quantities of high-affinity antibodies. This is the basis of the secondary immune response, which is typically so rapid that the pathogen is eliminated before the individual develops symptoms.
记忆B细胞并不活跃地分泌抗体，而是以静息状态留存在次级淋巴组织中。当相同抗原再次进入体内时，这些记忆B细胞迅速被重新激活，增殖并分化为浆细胞，产生大量高亲和力抗体。这就是二次免疫应答的基础，通常该应答极其迅速，以至于在个体出现症状之前病原体就已被消灭。
Correspondingly, memory T cells, both CD4⁺ and CD8⁺, persist in the body and can mount a swift secondary response. Memory T cells do not require co-stimulation for reactivation and are already present in higher frequencies than naive T cells, making the recall response extremely efficient. Vaccination exploits this principle by inducing the formation of memory cells without causing disease.
相应地，记忆T细胞（CD4⁺和CD8⁺）同样在体内持续存在，并能发起迅速的二次应答。记忆T细胞的再激活无需共刺激信号，且其基数远高于初始T细胞，因而回忆应答效率极高。疫苗接种正是利用这一原理，在不引起疾病的情况下诱导记忆细胞的形成。
9. Dendritic Cells: Professional Antigen-Presenting Cells | 树突状细胞：专职抗原呈递细胞
Dendritic cells (DCs) are the most potent antigen-presenting cells and are considered the sentinels of the immune system. They originate in the bone marrow, migrate into peripheral tissues and continuously sample their environment for foreign material.
树突状细胞（DCs）是最强效的抗原呈递细胞，被视为免疫系统的“哨兵”。它们来源于骨髓，迁移至外周组织，并持续对环境中的外来物质进行采样。
Immature dendritic cells capture antigens by phagocytosis or macropinocytosis. The encounter with pathogen-associated molecular patterns (PAMPs) through their pattern recognition receptors, such as Toll-like receptors, triggers their maturation. Mature DCs down-regulate their antigen-capture ability, up-regulate MHC I and MHC II expression and increase co-stimulatory molecules (B7). They then migrate via the lymphatic vessels to secondary lymphoid organs to present processed antigen to naive T cells.
未成熟的树突状细胞通过吞噬或巨胞饮作用捕获抗原。当模式识别受体（如Toll样受体）与病原相关分子模式（PAMPs）相遇后，DC开始成熟。成熟DC下调其抗原捕获能力，上调MHC I和MHC II分子的表达并增加共刺激分子（B7），然后沿淋巴管迁移至次级淋巴器官，将加工过的抗原呈递给初始T细胞。
Dendritic cells are unique in their ability to prime naive T cells, making them indispensable for initiating adaptive immune responses. They can cross-present extracellular antigens on MHC I molecules – a critical function that allows the immune system to generate CD8⁺ T cell responses against viruses that do not infect dendritic cells themselves.
树突状细胞具有独特的致敏初始T细胞的能力，因此对于启动适应性免疫应答不可或缺。它们可以将胞外抗原进行交叉呈递至MHC I类分子——这一关键功能使得免疫系统能够针对不感染DC本身的病毒产生CD8⁺ T细胞应答。
10. Natural Killer Cells and Other Leukocytes | 自然杀伤细胞及其他白细胞
Natural killer (NK) cells are large, granular lymphocytes that provide a rapid innate response against virally infected cells and tumour cells. Unlike CD8⁺ T cells, NK cells do not require prior sensitisation or antigen-specific receptors; they can recognise and kill target cells that have down-regulated MHC I expression, a common tactic used by viruses to evade T cells.
自然杀伤（NK）细胞是大颗粒淋巴细胞，能够对病毒感染细胞和肿瘤细胞做出快速的先天免疫应答。不同于CD8⁺ T细胞，NK细胞不需要预先致敏或抗原特异性受体；它们能够识别并杀死下调了MHC I类分子表达的靶细胞，而MHC I下调正是病毒逃避T细胞识别的常见策略。
NK cell activity is controlled by a balance of inhibitory and activating receptors. Healthy cells express normal levels of MHC I, engaging inhibitory receptors and preventing NK cell attack. When MHC I is lost, inhibition ceases and activating signals dominate, triggering degranulation and target cell apoptosis.
NK细胞的活性受抑制性受体和活化性受体的平衡调控。健康细胞表达正常水平的MHC I，与抑制性受体结合以防止NK细胞攻击。一旦MHC I丢失，抑制作用解除，活化信号占据主导，从而触发脱颗粒和靶细胞凋亡。
Other leukocytes include eosinophils, basophils and mast cells, which are primarily involved in defence against multicellular parasites and allergic reactions. Eosinophils release cytotoxic granule proteins onto large parasites, while basophils and mast cells release histamine, contributing to inflammation and the symptoms of allergy. Although not always emphasised in A-level, they round out the picture of innate effector cells.
其他白细胞还包括嗜酸性粒细胞、嗜碱性粒细胞和肥大细胞，它们主要参与对抗多细胞寄生虫和过敏反应。嗜酸性粒细胞能向大型寄生虫释放细胞毒性颗粒蛋白，而嗜碱性粒细胞和肥大细胞则释放组胺，引起炎症和过敏症状。虽然在A Level中不一定被重点强调，但它们使先天免疫效应细胞的全貌更加完整。
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