IGCSE Edexcel Biology: The Immune System | 免疫系统考点精讲

📚 IGCSE Edexcel Biology: The Immune System | 免疫系统考点精讲

The immune system is the body’s defence network against infectious diseases. Understanding how it works is essential for IGCSE Edexcel Biology. This article covers pathogens, non-specific defences, the specific immune response, memory cells, vaccination, and types of immunity, with clear explanations and key points to help you master the topic.

免疫系统是人体抵御传染病的防御网络。了解其工作原理对于 IGCSE Edexcel 生物考试至关重要。本文涵盖病原体、非特异性防御、特异性免疫应答、记忆细胞、疫苗接种和免疫类型，提供清晰的解释和关键要点，助你掌握这一主题。

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1. Pathogens: The Invaders | 病原体：入侵者

Pathogens are microorganisms that cause infectious diseases. They include bacteria, viruses, fungi, and protists. These pathogens can enter the body through various routes, such as cuts in the skin, inhalation of droplets, or ingestion of contaminated food and water. Once inside, they multiply and damage cells or release toxins, leading to the symptoms of disease.

病原体是引起传染病的微生物，包括细菌、病毒、真菌和原生生物。它们可以通过不同途径进入人体，例如皮肤伤口、吸入飞沫或摄入受污染的食物和水。一旦进入体内，它们就会繁殖并损伤细胞或释放毒素，从而引发疾病症状。

Not all microorganisms are harmful; many are beneficial or harmless. However, the body must distinguish between self and non-self to target harmful invaders. The immune system has evolved a multilayered defence strategy to recognise and eliminate pathogens efficiently.

并非所有微生物都是有害的；许多是有益的或无害的。然而，身体必须区分自身和外来物，以针对有害入侵者。免疫系统进化出了多层次的防御策略来有效识别并消灭病原体。

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2. First Line of Defence: Physical and Chemical Barriers | 第一道防线：物理与化学屏障

The body’s first line of defence consists of physical and chemical barriers that prevent pathogen entry. The skin is a tough, waterproof outer layer that blocks most microbes. Mucous membranes lining the respiratory tract secrete sticky mucus, which traps pathogens. Cilia, tiny hair-like structures, beat rhythmically to sweep the mucus and trapped pathogens out of the airways.

人体的第一道防线由阻止病原体进入的物理和化学屏障组成。皮肤是一层坚韧、防水的外层，能阻挡大多数微生物。呼吸道内壁的黏膜分泌黏性黏液，能够捕获病原体。纤毛是微小的毛发状结构，有节奏地摆动，将黏液和被捕获的病原体从呼吸道中扫除。

Chemical defences complement these physical barriers. The stomach produces hydrochloric acid (HCl), creating a highly acidic environment (pH around 2) that kills most pathogens ingested with food. Tears and saliva contain lysozyme, an enzyme that breaks down bacterial cell walls, providing an additional chemical shield. These non-specific mechanisms are always ready to act and do not distinguish between different types of pathogens.

化学防御是对物理屏障的补充。胃分泌盐酸（HCl），形成强酸性环境（pH 约 2），能杀灭随食物进入体内的大部分病原体。眼泪和唾液中含有溶菌酶，这种酶能破坏细菌细胞壁，提供额外的化学防护。这些非特异性机制时刻准备行动，且不区分病原体的类型。

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3. Phagocytosis: Non-Specific Defence | 吞噬作用：非特异性防御

If pathogens breach the first line of defence, a second line of non-specific cellular defence is activated. Phagocytes, a type of white blood cell such as neutrophils and macrophages, carry out phagocytosis. They recognise foreign particles through general surface receptors and respond rapidly to any invasion.

如果病原体突破了第一道防线，第二道非特异性细胞防御就会被激活。吞噬细胞是一类白细胞，如中性粒细胞和巨噬细胞，执行吞噬作用。它们通过通用的表面受体识别外来颗粒，并对任何入侵做出快速响应。

During phagocytosis, the phagocyte moves towards the pathogen, attracted by chemical signals (chemotaxis). It then engulfs the pathogen by extending its cell membrane around it, forming a vesicle called a phagosome. Lysosomes within the phagocyte fuse with the phagosome and release digestive enzymes that break down the pathogen. Finally, the digested debris is expelled from the cell. This process does not provide long-term immunity but is crucial for immediate defence.

在吞噬作用过程中，吞噬细胞受化学信号吸引而向病原体移动（趋化作用）。然后，它伸出细胞膜将病原体包裹起来，形成一个称为吞噬体的小泡。吞噬细胞内的溶酶体与吞噬体融合，释放消化酶，分解病原体。最后，消化后的残渣被排出细胞外。这一过程不提供长期免疫，但对于即时防御至关重要。

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4. The Specific Immune Response: An Overview | 特异性免疫应答：概览

When non-specific defences fail to eliminate all pathogens, the specific immune response is triggered. This response involves lymphocytes (a type of white blood cell) and is highly specific to particular antigens found on the surface of each pathogen. The specific immune system can distinguish between different pathogens and provides targeted destruction along with immunological memory.

当非特异性防御未能消灭所有病原体时，特异性免疫应答就会被触发。该应答涉及淋巴细胞（一种白细胞），并对每种病原体表面的特定抗原具有高度特异性。特异性免疫系统能够区分不同的病原体，提供有针对性的破坏并建立免疫记忆。

The specific immune response has two main arms: the humoral response, mediated by B lymphocytes (B cells) that produce antibodies, and the cell-mediated response, involving T lymphocytes (T cells) that destroy infected cells directly. This coordination ensures that both free-floating pathogens and cells already invaded are tackled effectively.

特异性免疫应答包含两个主要分支：由 B 淋巴细胞（B 细胞）介导的体液免疫，负责产生抗体；以及由 T 淋巴细胞（T 细胞）参与的细胞免疫，直接破坏受感染的细胞。这种协同作用确保了对游离的病原体和已入侵细胞的病原体都能有效应对。

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5. Antigens: Identifying the Enemy | 抗原：识别敌人

Antigens are molecules, typically proteins or polysaccharides, present on the outer surface of pathogens. Each antigen has a unique three-dimensional shape that acts as a molecular signature. The immune system uses these antigens to distinguish ‘self’ from ‘non-self’. Our own cells carry self-antigens, which are normally ignored by the immune system to prevent an attack on healthy tissue.

抗原是存在于病原体外表面的分子，通常是蛋白质或多糖。每种抗原都具有独特的三维形状，就像一份分子签名。免疫系统利用这些抗原区分“自己”与“非己”。我们自身的细胞带有自体抗原，免疫系统通常会忽略它们，以免攻击健康组织。

When a foreign antigen is detected, specific lymphocytes that have complementary receptor proteins on their surface bind to it. This binding is highly specific, much like a lock and key. The recognition event kicks off the immune cascade that leads to the destruction of the pathogen and the formation of memory cells.

当检测到外来抗原时，表面带有互补受体蛋白的特异性淋巴细胞会与之结合。这种结合具有高度特异性，类似于锁和钥匙的关系。识别事件会启动免疫级联反应，最终导致病原体被消灭，并形成记忆细胞。

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6. Lymphocytes and Antibody Production | 淋巴细胞与抗体产生

B lymphocytes (B cells) are central to antibody production. Each B cell has thousands of receptor molecules on its membrane, each specific to one particular antigen. When a B cell encounters its complementary antigen — often with help from T helper cells — it becomes activated. This activation triggers clonal expansion: the B cell divides rapidly by mitosis, producing a large population of identical cells.

B 淋巴细胞（B 细胞）是抗体产生的核心。每个 B 细胞的细胞膜上都有数千个受体分子，每一个都特异于某一种抗原。当 B 细胞遇到其互补抗原时（通常在辅助 T 细胞的帮助下），就会被激活。这种激活会引发克隆扩增：B 细胞通过有丝分裂迅速分裂，产生大量完全相同的细胞。

Most of these daughter cells differentiate into plasma cells, which are antibody factories. Plasma cells synthesise and secrete huge quantities of antibodies into the blood and lymph. Antibodies are Y-shaped proteins (immunoglobulins) that bind specifically to the antigen that triggered their production. They can neutralise toxins, clump pathogens together (agglutination), or mark them for destruction by phagocytes.

这些子细胞中的大多数会分化为浆细胞，即抗体工厂。浆细胞合成大量抗体并分泌到血液和淋巴中。抗体是 Y 形蛋白质（免疫球蛋白），能特异性地与触发其产生的抗原结合。它们可以中和毒素，凝集病原体（凝集作用），或者标记病原体供吞噬细胞消灭。

A small proportion of activated B cells become memory B cells rather than plasma cells. These cells do not secrete antibodies but remain in the body for years, ready to respond rapidly if the same antigen appears again. This is the foundation of long-lasting immunity.

一小部分活化的 B 细胞会变成记忆 B 细胞，而不是浆细胞。这些细胞不分泌抗体，但可以在体内存活多年，随时准备在相同抗原再次出现时做出快速应答。这就是持久免疫的基础。

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7. Primary and Secondary Immune Responses | 初次与再次免疫应答

The first time the body is exposed to a specific pathogen, the primary immune response is initiated. There is a lag phase of several days during which the specific B cells are selected and cloned, and plasma cells begin to produce antibodies. During this lag, the pathogen can multiply to high levels, and symptoms of the disease often appear. The antibody concentration in the blood rises slowly and peaks after a week or two, then gradually declines.

身体第一次接触特定病原体时，会启动初次免疫应答。存在数天的滞后阶段，在此期间特异性 B 细胞被筛选和克隆，浆细胞开始产生抗体。在滞后期内，病原体可能大量繁殖，常常出现疾病症状。血液中的抗体浓度缓慢上升，约一两周后达到峰值，随后逐渐下降。

If the same pathogen infects the body again, a secondary immune response occurs. This response is much faster (lag of only hours) and produces a much higher concentration of antibodies. The reason is that memory B and T cells generated during the primary response are already present and can rapidly differentiate into plasma cells and effector T cells. The pathogen is often destroyed before any symptoms develop, conferring long-term immunity.

如果同一病原体再次感染人体，就会发生再次免疫应答（二次应答）。这次应答快得多（滞后仅数小时），并产生浓度更高的抗体。原因在于初次应答中产生的记忆 B 细胞和记忆 T 细胞已经存在于体内，可以迅速分化成浆细胞和效应 T 细胞。病原体往往在症状出现之前就被消灭，从而赋予长期免疫力。

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8. Memory Cells: The Key to Long-Term Immunity | 记忆细胞：长期免疫的关键

Memory cells are the cellular basis of immunological memory. Both B and T lymphocytes can differentiate into long-lived memory cells during an immune response. These cells have a much longer lifespan than plasma cells — often surviving for decades, or even a lifetime. They continuously patrol the blood and lymphatic system, remaining quiescent until they encounter their specific antigen.

记忆细胞是免疫记忆的细胞基础。在免疫应答中，B 淋巴细胞和 T 淋巴细胞都可以分化为长寿的记忆细胞。这些细胞的寿命远长于浆细胞——往往存活数十年，甚至终生。它们持续在血液和淋巴系统中巡逻，保持静息状态，直到遇到其特异抗原。

Upon re-exposure, memory cells mount a rapid and robust response. Memory B cells quickly proliferate and differentiate into antibody-secreting plasma cells. Memory T cells, including helper and killer T cells, become active immediately. This explains why many infectious diseases, such as measles or chickenpox, only strike once — the immune system retains a pool of memory cells that block future infections.

再次接触抗原时，记忆细胞会发动快速且强烈的应答。记忆 B 细胞迅速增殖并分化为分泌抗体的浆细胞。记忆 T 细胞，包括辅助 T 细胞和杀伤 T 细胞，会立即活化。这就解释了为什么许多传染病，如麻疹或水痘，只会感染一次——免疫系统保留了一群记忆细胞来阻断未来的感染。

The presence of memory cells forms the rationale behind vaccination. By deliberately inducing the production of memory cells without causing disease, vaccines prepare the immune system for future encounters.

记忆细胞的存在构成了疫苗接种的原理基础。疫苗通过在不引发疾病的情况下诱导产生记忆细胞，从而使免疫系统为未来的入侵做好准备。

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9. Vaccination: Artificial Active Immunity | 疫苗接种：人工主动免疫

A vaccine is a preparation containing antigens derived from a pathogen. These may be inactivated (killed) pathogens, live attenuated (weakened) strains, or purified antigen fragments (like spike proteins). The vaccine is introduced into the body, usually by injection, but sometimes orally. Because the vaccine mimics the pathogen without causing disease, it triggers a primary immune response, including the production of memory cells.

疫苗是含有病原体抗原的制剂。可以是灭活（死亡）的病原体、减毒活疫苗，或是纯化的抗原片段（如刺突蛋白）。疫苗通常通过注射引入体内，有时也口服。由于疫苗模拟了病原体而不致病，它会引发初次免疫应答，包括产生记忆细胞。

Later, if the individual is exposed to the real pathogen, memory cells recognise the antigens and generate a powerful secondary response that eliminates the invader before it can cause illness. This process is called artificial active immunity because the body actively produces its own antibodies and memory cells in response to a harmless antigenic stimulus. Vaccination has been crucial in controlling diseases like polio, tetanus, and measles.

之后，如果个体接触到真正的病原体，记忆细胞会识别抗原，并产生强大的再次应答，在病原体致病之前将其消灭。这一过程被称为人工主动免疫，因为身体针对无害的抗原刺激，主动产生了自身的抗体和记忆细胞。疫苗接种在控制脊髓灰质炎、破伤风和麻疹等疾病方面发挥了关键作用。

Booster vaccinations are sometimes needed to ‘remind’ the immune system, as the memory cell population for certain pathogens may decline over time. Boosters re‑expose the body to antigens, amplifying the number of memory cells and ensuring continued protection.

有时需要加强免疫来“提醒”免疫系统，因为针对某些病原体的记忆细胞数量可能会随时间的推移而下降。加强针让身体再次接触抗原，从而扩大记忆细胞的群体数量，确保持续保护。

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10. Passive Immunity: Natural and Artificial | 被动免疫：天然与人工

Passive immunity involves the acquisition of pre-made antibodies from an external source, rather than the body making its own. This provides immediate, but short‑lived, protection because the antibodies are eventually broken down, and no memory cells are produced. Passive immunity does not confer long‑term protection.

被动免疫是指从外部来源获取现成的抗体，而不是身体自身产生。这能提供即时但短暂的保护，因为抗体最终会被分解，而且不产生记忆细胞。被动免疫不能提供长期保护。

Natural passive immunity occurs when antibodies cross the placenta from mother to fetus during pregnancy, or when a baby receives antibodies through breast milk (especially colostrum). This provides the newborn with crucial protection during the first months of life while the infant’s own immune system matures.

天然被动免疫发生在怀孕期间抗体通过胎盘从母体传递给胎儿时，或者婴儿通过母乳（尤其是初乳）获得抗体时。这为新生儿在生命最初几个月提供至关重要的保护，同时婴儿自身的免疫系统也在不断成熟。

Artificial passive immunity is achieved by injecting antibodies obtained from another organism. For example, antivenom against snake venom is produced by injecting a small, non‑lethal dose of venom into an animal such as a horse, collecting the resulting antibodies, and purifying them for human use. This type of immunity is used for immediate treatment of life‑threatening infections or toxin exposure, but protection wanes after a few weeks.

人工被动免疫通过注射从其他生物体获得的抗体来实现。例如，抗蛇毒血清是将小剂量非致死性蛇毒注射到马等动物体内，收集所产生的抗体并纯化后供人使用。这类免疫用于即刻治疗危及生命的感染或毒素暴露，但保护作用会在几周后减弱。

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11. Comparing Active and Passive Immunity | 主动与被动免疫比较

Active immunity and passive immunity differ fundamentally in their origin, duration, and whether memory cells are involved. Active immunity results from the body’s own adaptive immune response. It takes time to develop (days to weeks) but provides long‑lasting protection because memory cells are formed. This immunity can be acquired naturally through infection, or artificially through vaccination.

主动免疫和被动免疫在来源、持续时间以及是否涉及记忆细胞等方面存在根本区别。主动免疫源自人体自身的适应性免疫应答。它需要时间（数天至数周）才能建立，但由于形成了记忆细胞，能提供持久的保护。这种免疫可以通过感染天然获得，也可以通过疫苗接种人工获得。

Passive immunity, on the other hand, provides immediate protection by transferring antibodies from another source. No memory cells are generated, so the protection is temporary, typically lasting only a few weeks or months. It can be acquired naturally from a mother or artificially through injected antibodies. The table below summarises the key comparisons.

另一方面，被动免疫通过转移来自其他来源的抗体提供即时保护。由于不产生记忆细胞，保护是暂时的，通常只能持续数周或数月。它可以从母亲处天然获得，也可以通过注射抗体人工获得。下表总结了关键的比较。

Feature | 特点	Active Immunity | 主动免疫	Passive Immunity | 被动免疫
Source of antibodies | 抗体来源	Produced by own body | 自身产生	Received from external source | 从外部获取
Onset of protection | 保护开始	Slow (days–weeks) | 慢（数天至数周）	Immediate | 即时
Duration | 持续时间	Long-lasting (years to lifetime) | 持久（数年至终生）	Short-term (weeks to months) | 短期（数周至数月）
Memory cells produced | 产生记忆细胞	Yes | 是	No | 否
Natural example | 天然示例	Recovery from infection | 感染康复	Antibodies via placenta/breast milk | 胎盘/母乳传递抗体
Artificial example | 人工示例	Vaccination | 疫苗接种	Injection of antivenom or antibody serum | 注射抗蛇毒血清或抗体血清

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12. Herd Immunity and Public Health | 群体免疫与公共卫生

Herd immunity is a form of indirect protection that occurs when a sufficiently high proportion of the population becomes immune to an infectious disease, either through vaccination or previous infection. When immune individuals are numerous, the chain of pathogen transmission is disrupted, making

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