📚 IGCSE WJEC Biology: The Immune System Revision Guide | IGCSE WJEC 生物：免疫系统备考指南
The immune system is a complex network of cells, tissues and organs that defends the body against pathogens such as bacteria, viruses, fungi and parasites. For IGCSE WJEC Biology, you need to understand how non-specific and specific defences work together, how antibodies are produced, the difference between active and passive immunity, and the importance of vaccination in public health. This revision guide covers all key exam points with clear explanations in both English and Chinese.
免疫系统是一个由细胞、组织和器官构成的复杂网络，负责抵御细菌、病毒、真菌和寄生虫等病原体。在 IGCSE WJEC 生物考试中，你需要理解非特异性防御与特异性防御如何协同运作，抗体如何产生，主动免疫与被动免疫之间的区别，以及疫苗接种对公共卫生的重要性。这份备考指南用中英双语清晰讲解所有核心考点。
1. Pathogens and Infectious Diseases | 病原体与传染病
A pathogen is a microorganism that causes disease. Common pathogens include bacteria, viruses, fungi and protists. They can be spread by direct contact, airborne droplets, contaminated food or water, and vectors such as mosquitoes.
病原体是引起疾病的微生物。常见的病原体包括细菌、病毒、真菌和原生生物。它们可通过直接接触、飞沫传播、受污染的食物或水，以及蚊虫等媒介传播。
When pathogens enter the body, they can damage cells directly or produce toxins. The immune system works to recognise these foreign invaders and eliminate them before they cause serious harm.
当病原体进入人体时，它们可直接破坏细胞或产生毒素。免疫系统负责识别这些外来入侵者，并在它们造成严重伤害之前将其消灭。
2. Non-specific Defences: Physical and Chemical Barriers | 非特异性防御：物理与化学屏障
The body’s first line of defence is non-specific, meaning it acts against all pathogens in the same way. Physical barriers include the skin, which is a tough, impermeable layer, and the mucous membranes lining the respiratory and digestive tracts.
人体的第一道防线是非特异性的，这意味着它以同样的方式对抗所有病原体。物理屏障包括坚韧且不渗透的皮肤，以及衬覆在呼吸道和消化道内的黏膜。
Chemical barriers also play a vital role. Tears contain lysozyme, an enzyme that breaks down bacterial cell walls. Stomach acid (hydrochloric acid) kills most ingested pathogens, and sebum on the skin has antimicrobial properties.
化学屏障同样至关重要。泪液含有溶菌酶，可分解细菌细胞壁。胃酸（盐酸）能杀死大部分摄入的病原体，而皮肤上的皮脂具有抗微生物特性。
Mucus in the airways traps microorganisms, and cilia on the surface of epithelial cells then sweep the mucus upwards to be swallowed or expelled. This is why coughing and sneezing are protective reflexes.
气道中的黏液会黏住微生物，上皮细胞表面的纤毛随后会将黏液向上扫动，以便吞下或排出。这就是为什么咳嗽和打喷嚏属于保护性反射。
3. Phagocytosis: The Second Line of Defence | 吞噬作用：第二道防线
If pathogens breach the first barriers, the non-specific second line of defence activates. Phagocytosis is a key process carried out by white blood cells called phagocytes, such as macrophages and neutrophils.
如果病原体突破了第一道屏障，非特异性第二道防线就会启动。吞噬作用是由巨噬细胞和中性粒细胞等吞噬细胞执行的关键过程。
During phagocytosis, the phagocyte detects chemical signals from the pathogen or damaged tissue, moves towards it, and engulfs it by extending pseudopodia around the microbe. The pathogen is then enclosed in a phagosome.
吞噬过程中，吞噬细胞探测到来自病原体或受损组织的化学信号，向其移动，并通过伸出伪足将微生物包裹。病原体随后被包裹在吞噬体中。
The phagosome fuses with lysosomes containing digestive enzymes and hydrogen peroxide, which break down the pathogen. The harmless products are then absorbed or released by exocytosis.
吞噬体与含有消化酶和过氧化氢的溶酶体融合，这些物质将病原体分解。无害产物随后被吸收或通过胞吐作用释放。
Phagocytosis is non-specific because phagocytes do not distinguish between different types of pathogen; they engulf any foreign particle that is marked for destruction.
吞噬作用是非特异性的，因为吞噬细胞不区分不同类型的病原体；它们会吞噬任何被标记为需要清除的外来颗粒。
4. The Specific Immune Response: Lymphocytes and Antibodies | 特异性免疫应答：淋巴细胞与抗体
If a pathogen overcomes non-specific defences, the specific immune response is triggered. This involves lymphocytes, a type of white blood cell. There are two main types: B lymphocytes (B cells) and T lymphocytes (T cells).
如果病原体突破了非特异性防御，就会触发特异性免疫应答。这涉及淋巴细胞这一种白细胞。主要有两种类型：B 淋巴细胞（B 细胞）和 T 淋巴细胞（T 细胞）。
B cells are responsible for producing antibodies. Each B cell has receptor molecules on its surface that can bind to a specific antigen, which is a molecule on the surface of a pathogen that the immune system recognises as foreign.
B 细胞负责产生抗体。每个 B 细胞表面都有受体分子，可与一种特定的抗原结合。抗原是病原体表面的一种分子，被免疫系统识别为外来物质。
When a B cell binds to its complementary antigen, it becomes activated and divides rapidly by mitosis to form plasma cells and memory cells. Plasma cells secrete large quantities of antibodies into the blood and lymph.
当 B 细胞与互补的抗原结合时，它便会被激活，并经过有丝分裂迅速增殖，形成浆细胞和记忆细胞。浆细胞将大量抗体分泌到血液和淋巴中。
Antibodies are Y-shaped proteins that have a specific binding site complementary to the antigen. They work by clumping pathogens together (agglutination), neutralising toxins, and marking pathogens for phagocytosis.
抗体是一种 Y 形蛋白质，具有与抗原互补的特异性结合位点。它们通过使病原体凝集（凝集反应）、中和毒素以及标记病原体以供吞噬来发挥作用。
5. Antigens and Antibody Specificity | 抗原与抗体特异性
Antigens are usually large protein or polysaccharide molecules on the surface of cells, viruses or toxins. The immune system can distinguish ‘self’ from ‘non-self’ by recognising antigens that do not belong to the body’s own cells.
抗原通常是位于细胞、病毒或毒素表面的大分子蛋白质或多糖。免疫系统可通过识别不属于人体自身细胞的抗原，来区分’自己’与’非己’。
Each antibody molecule has a unique antigen-binding site that is complementary in shape to a specific antigen, rather like a lock and key. This ensures that an antibody against the flu virus will not bind to a bacterial pathogen.
每个抗体分子都有一个独特的抗原结合位点，其形状与特定抗原互补，就像锁和钥匙一样。这确保了针对流感病毒的抗体不会与细菌性病原体结合。
There is an enormous diversity of lymphocytes, so the body can produce antibodies against millions of different antigens. This ability is generated by random gene rearrangements during lymphocyte development.
淋巴细胞的种类极其多样，因此人体能够针对数百万种不同的抗原产生抗体。这种能力源自淋巴细胞发育过程中随机的基因重排。
6. Primary and Secondary Immune Responses | 初次与二次免疫应答
When the body encounters a pathogen for the first time, the primary immune response occurs. It is slow because it takes time for the specific B cell with the correct receptor to be found and activated, and for plasma cells to produce antibodies.
当人体第一次遭遇某种病原体时，会发生初次免疫应答。该应答速度较慢，因为找到并激活具有正确受体的特定 B 细胞，以及让浆细胞产生抗体都需要时间。
During the primary response, the concentration of antibodies in the blood rises slowly, typically peaking after about 10–14 days. The person may develop symptoms of the disease while the immune system fights the infection.
在初次应答中，血液抗体浓度上升缓慢，通常在 10–14 天后达到峰值。当免疫系统对抗感染时，患者可能会出现疾病症状。
After the infection is cleared, some B cells become memory cells that remain in the body for years or even a lifetime. If the same pathogen enters again, the secondary immune response is triggered.
感染被清除后，部分 B 细胞会转变为记忆细胞，在体内存留多年甚至终生。如果相同的病原体再次入侵，就会触发二次免疫应答。
The secondary response is much faster and stronger. Memory cells recognise the antigen immediately and divide rapidly into plasma cells, producing a large amount of antibody within hours, often preventing symptoms entirely.
二次应答速度更快、强度更高。记忆细胞立即识别抗原并迅速分裂为浆细胞，在数小时内产生大量抗体，往往能完全阻止症状出现。
Antibody level (arbitrary units) in primary response ≈ slow rise, low peak; secondary response ≈ rapid rise, high peak
初次应答抗体水平（任意单位）≈ 缓慢上升、峰值低；二次应答 ≈ 快速上升、峰值高
7. Vaccination and Immunological Memory | 疫苗接种与免疫记忆
Vaccination is the administration of a harmless form of a pathogen or its parts to stimulate an immune response without causing disease. Vaccines may contain dead or inactivated pathogens, weakened (attenuated) strains, or isolated antigens.
疫苗接种是指将无害形式的病原体或其组成部分注入体内，以激发免疫应答而不引起疾病。疫苗可能含有灭活的病原体、减毒株或分离的抗原。
Because the vaccine antigens are still recognised as foreign, the immune system mounts a primary response and produces memory cells. If the person later encounters the real pathogen, a rapid secondary response prevents illness.
由于疫苗抗原仍被识别为外来物质，免疫系统会进行初次应答并产生记忆细胞。如果此人日后接触到真正的病原体，快速的二次应答便将疾病拒之门外。
Vaccination has led to the global eradication of smallpox and has dramatically reduced the incidence of diseases such as polio, measles and tetanus. It is one of the most successful public health interventions in history.
疫苗接种已在全球范围内消灭了天花，并大幅降低了脊髓灰质炎、麻疹和破伤风等疾病的发病率。这是历史上最成功的公共卫生干预措施之一。
8. Active and Passive Immunity | 主动免疫与被动免疫
Active immunity results when the body makes its own antibodies and memory cells after being exposed to an antigen. This can occur naturally through infection or artificially through vaccination. It provides long-term protection because immunological memory is established.
主动免疫是指人体在接触抗原后自行产生抗体和记忆细胞。这可通过自然感染或人工接种疫苗实现。由于产生了免疫记忆，它可提供长期保护。
Passive immunity is acquired without the immune system having to produce antibodies. Antibodies are received from an external source, such as from mother to baby across the placenta or in breast milk (natural passive), or through injection of antibody serum (artificial passive).
被动免疫获取时，免疫系统无需自行产生抗体。抗体从外部来源获取，例如通过胎盘或母乳从母体传给婴儿（自然被动），或通过注射抗体血清（人工被动）。
Passive immunity provides immediate protection but is short-lived because no memory cells are formed and the injected antibodies are gradually broken down. It is useful in emergencies, such as after exposure to tetanus or snake venom.
被动免疫提供即时保护，但作用短暂，因为没有形成记忆细胞，且注入的抗体逐渐被分解。这在紧急情况下很有用，例如在接触破伤风或蛇毒后。
| Feature / 特征 | Active Immunity / 主动免疫 | Passive Immunity / 被动免疫 |
|---|---|---|
| Source of antibodies / 抗体来源 | Made by the body / 自身产生 | Received from outside / 外部获取 |
| Onset of protection / 保护出现速度 | Slow (days to weeks) / 慢 (数天至数周) | Immediate / 即时 |
| Duration / 持续时间 | Long-term, often lifelong / 长期，常终生 | Short-term (weeks to months) / 短期 (数周至数月) |
| Memory cells / 记忆细胞 | Formed / 形成 | Not formed / 不形成 |
| Examples / 例子 | Recovering from chickenpox, vaccination / 水痘康复, 接种疫苗 | Breastfeeding, antivenom injection / 母乳喂养, 注射抗毒血清 |
9. Herd Immunity and Public Health | 群体免疫与公共卫生
Herd immunity occurs when a large proportion of a population is immune to a disease, usually through vaccination, which reduces the spread of the pathogen. Even individuals who are not immune, such as newborns or people with medical exemptions, are protected because the pathogen cannot find enough susceptible hosts.
当人群中的很大一部分人对某种疾病具有免疫力（通常通过疫苗接种）时，就会形成群体免疫，从而减少病原体的传播。即使是没有免疫力的个体，如新生儿或因医学原因不能接种者，也会受到保护，因为病原体找不到足够的易感宿主。
To achieve herd immunity against highly contagious diseases like measles, over 90–95% of the population needs to be vaccinated. If vaccination rates fall, outbreaks can occur, as seen in some communities with low vaccine uptake.
对于麻疹等高度传染性疾病，需要超过 90%–95% 的人口接种疫苗才能实现群体免疫。如果接种率下降，就可能暴发疫情，这在一些疫苗接种率较低的社群中已有先例。
Vaccination programmes not only protect individuals but also contribute to the eradication of infectious diseases. The success of such programmes depends on public trust and high coverage rates.
免疫接种计划不仅能保护个体，还有助于根除传染病。这类计划的成功依赖于公众信任和高覆盖率。
10. Immune System Disorders: Allergies and HIV | 免疫系统失调：过敏与 HIV
Sometimes the immune system overreacts to harmless substances, causing allergies. Common allergens include pollen, dust mites and certain foods. The reaction involves a type of antibody called IgE and the release of histamine from mast cells, leading to symptoms such as sneezing, rashes and swelling.
有时免疫系统会对无害物质过度反应，引发过敏。常见过敏原包括花粉、尘螨和某些食物。该反应涉及一种叫做 IgE 的抗体，以及肥大细胞释放组胺，从而导致打喷嚏、皮疹和肿胀等症状。
Severe allergic reactions, known as anaphylaxis, can cause breathing difficulties and a rapid drop in blood pressure, requiring emergency treatment with adrenaline (epinephrine).
严重的过敏反应，称为过敏性休克，可导致呼吸困难和血压迅速下降，需要紧急使用肾上腺素进行治疗。
Human immunodeficiency virus (HIV) attacks the immune system directly. It infects and destroys helper T cells, which are critical for coordinating the immune response. Over time, this leads to acquired immunodeficiency syndrome (AIDS), where the body becomes vulnerable to opportunistic infections and cancers.
人类免疫缺陷病毒 (HIV) 直接攻击免疫系统。它感染并破坏辅助 T 细胞，这些细胞对协调免疫应答至关重要。久而久之，就会导致获得性免疫缺陷综合征 (AIDS)，使人体变得容易受到机会性感染和癌症的侵袭。
Understanding how HIV weakens the immune system highlights the importance of each component of our defences and explains why even a minor infection can become life-threatening in someone with AIDS.
了解 HIV 如何削弱免疫系统，能突显出我们防御体系中每个组成部分的重要性，也解释了为什么在艾滋病患者身上，即使是轻微感染也可能危及生命。
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