OCR ALevel Biology Disease 免疫系统 考点详解
疾病与免疫是 OCR A-Level Biology 的重要模块,也是历年考试的高频出题区域。考生需要理解病原体的种类与传播方式、人体三道防线如何协同工作、特异性免疫中 T 细胞与 B 细胞的分工,以及疫苗和抗生素背后的原理。本文按考纲逻辑逐层拆解,中英对照,配合实例与答题模板,帮助备考 OCR Biology A (H420) 的同学把这一章真正吃透。
Disease and immunity are a core module in OCR A-Level Biology and a favourite source of exam questions. You need to understand the types and transmission of pathogens, how the three lines of defence work together, how T cells and B cells divide the labour in the specific immune response, and the biology behind vaccines and antibiotics. This article breaks the topic down layer by layer in Chinese and English, with examples and answer frameworks, to help students preparing for OCR Biology A (H420) master the chapter properly.
1. 病原体的主要类型 Major Types of Pathogens
病原体(pathogen)是能引起疾病的微生物,考试常考四类。细菌(bacteria)是单细胞原核生物,没有核膜,可在细胞外独立繁殖,例如引起结核病的结核分枝杆菌(Mycobacterium tuberculosis)。病毒(virus)比细菌小得多,由核酸与蛋白质外壳组成,必须寄居在宿主细胞内借助宿主核糖体复制,例如 HIV 和流感病毒。真菌(fungi)是真核生物,如引起足癣的皮肤癣菌。原生生物(protoctista)如疟原虫(Plasmodium)也是真核生物,疟原虫需在蚊子和人体内交替完成生活史。
A pathogen is a microorganism that causes disease, and exams often test four groups. Bacteria are single-celled prokaryotes with no nuclear membrane; they can reproduce independently outside cells, for example Mycobacterium tuberculosis, which causes tuberculosis. Viruses are much smaller than bacteria and consist of nucleic acid wrapped in a protein coat; they must live inside host cells and use host ribosomes to replicate, for example HIV and influenza viruses. Fungi are eukaryotic organisms, such as the skin fungi that cause athlete's foot. Protoctista such as Plasmodium are also eukaryotic; Plasmodium must alternate between mosquitoes and humans to complete its life cycle.
2. 传播途径:病原体如何扩散 How Pathogens Spread
切断传播途径是预防疾病的关键,因此传播方式是选择题与简答题的常客。飞沫传播(droplet infection):咳嗽或打喷嚏喷出的含菌飞沫被他人吸入,结核病与流感都以此传播。直接接触:皮肤接触或性接触传播,如 HIV。间接接触:接触被污染的物品(fomite,污染物)。粪口传播:污水或受污染的食物和水,引发腹泻类疾病。病媒传播(vector-borne):病原体借助动物媒介传播,如雌性按蚊传播疟疾。植物病害还可用风、昆虫和土壤传播。答题时注意区分:传播途径针对病原体如何到达新宿主,感染部位则说明病原体在体内何处繁殖,两者不要混淆。
Breaking the routes of transmission is the key to preventing disease, so this topic appears constantly in multiple-choice and short-answer questions. Droplet infection: infected droplets sprayed out by coughing or sneezing are inhaled by other people; tuberculosis and influenza spread this way. Direct contact: skin-to-skin or sexual contact, for example HIV. Indirect contact: touching contaminated objects (fomites). Faecal-oral transmission: sewage, contaminated food and water cause diarrhoeal diseases. Vector-borne transmission: the pathogen is carried by an animal vector, for example female Anopheles mosquitoes transmitting malaria. Plant diseases may spread by wind, insects and soil. When answering, be careful: transmission route describes how the pathogen reaches a new host, while the site of infection describes where it multiplies inside the body; do not mix the two up.
3. 第一道防线与第二道防线 First and Second Lines of Defence
人体非特异性防御(non-specific defence)不分敌我,对所有病原体一视同仁,又分为两道。第一道防线是物理与化学屏障:皮肤最外层由角蛋白(keratin)构成,汗液中的溶菌酶(lysozyme)可破坏细菌细胞壁;呼吸道内壁的黏液(mucus)粘住病原体,由纤毛(cilia)向上摆动形成"黏液纤毛电梯"将其清除;胃酸的高 pH 环境杀灭随食物进入的微生物。第二道防线是炎症反应(inflammatory response):受损组织释放组胺(histamine),引起小动脉扩张和毛细血管通透性增加,使局部发红发热、肿胀疼痛,大量吞噬细胞随血液到达感染部位,发热还抑制部分病原体繁殖。这两道防线都属于先天免疫,无需接触过病原体即可生效。
Non-specific defence treats every pathogen in the same way and is divided into two lines. The first line is physical and chemical barriers: the outer layer of skin is made of keratin; lysozyme in sweat destroys bacterial cell walls; mucus lining the airways traps pathogens and cilia beat upwards to remove them like a mucociliary escalator; stomach acid kills microorganisms entering with food. The second line is the inflammatory response: damaged tissues release histamine, which dilates small arteries and increases capillary permeability, so the area becomes red, hot, swollen and painful, and large numbers of phagocytes travel in the blood to the site of infection; fever also slows the reproduction of some pathogens. Both lines are innate immunity: they work without any previous exposure to the pathogen.
4. 吞噬作用:非特异性细胞防御 Phagocytosis
吞噬细胞包括中性粒细胞(neutrophils)和巨噬细胞(macrophages),它们通过吞噬作用(phagocytosis)消灭病原体。过程分五步:识别并靠近病原体;病原体表面的化学信号被吞噬细胞表面的受体识别;细胞膜内陷,将病原体包裹形成吞噬体(phagosome);溶酶体(lysosome)与吞噬体融合,释放水解酶将病原体消化;消化后的碎片被排出或作为抗原呈递到细胞表面。OCR 常考排序题或流程图题,建议把五个关键词按顺序默写:approach, engulf, fuse, digest, present。巨噬细胞消化病原体后把抗原片段展示在细胞表面,这正好把第二道防线和第三道防线衔接起来。
Phagocytes include neutrophils and macrophages, which destroy pathogens by phagocytosis. The process has five steps: recognising and moving towards the pathogen; receptors on the phagocyte surface binding to chemical signals on the pathogen; the cell membrane folding inwards to surround the pathogen, forming a phagosome; lysosomes fusing with the phagosome and releasing hydrolytic enzymes to digest the pathogen; and the debris being egested or presented as antigens on the cell surface. OCR often sets sequencing or flow-diagram questions, so practise writing the five keywords in order: approach, engulf, fuse, digest, present. After digesting a pathogen, macrophages display antigen fragments on their surface, which links the second line of defence to the third.
5. 特异性免疫:T 细胞与 B 细胞的分工 The Specific Immune Response
第三道防线是特异性免疫,分为细胞免疫与体液免疫两条通路。细胞免疫(cell-mediated immunity)由 T 淋巴细胞执行:巨噬细胞将抗原呈递给辅助性 T 细胞(helper T cells),激活的辅助性 T 细胞分泌细胞因子(cytokines),刺激细胞毒性 T 细胞(cytotoxic T cells)增殖;细胞毒性 T 细胞通过释放穿孔素等物质杀死被病原体感染的宿主细胞,对病毒性疾病尤其重要。体液免疫(humoral immunity)由 B 淋巴细胞执行:B 细胞表面的抗体与抗原结合后被激活,增殖分化为浆细胞(plasma cells)和记忆细胞(memory cells);浆细胞大量分泌抗体进入血液,抗体与病原体结合形成抗原抗体复合物,使其凝集并被吞噬细胞清除。OCR 高频考点是区分:细胞免疫对付"已被感染的细胞",体液免疫对付"游离在体液中的病原体"。
The third line of defence is the specific immune response, which splits into cell-mediated and humoral pathways. Cell-mediated immunity is carried out by T lymphocytes: macrophages present antigens to helper T cells, activated helper T cells secrete cytokines and stimulate cytotoxic T cells to divide; cytotoxic T cells kill infected host cells by releasing substances such as perforin, which matters most against viral disease. Humoral immunity is carried out by B lymphocytes: antibodies on the B cell surface bind to antigens, activating the B cell to divide into plasma cells and memory cells; plasma cells secrete large amounts of antibody into the blood, and antibodies bind to pathogens to form antigen-antibody complexes, causing agglutination so phagocytes can clear them. The classic OCR question distinguishes the two: cell-mediated immunity deals with infected cells, while humoral immunity deals with free pathogens in the body fluids.
6. 疫苗、主动免疫与被动免疫 Vaccination, Active and Passive Immunity
疫苗(vaccine)含有灭活或减毒的病原体或其抗原成分,注射后引起初次免疫应答:产生浆细胞和记忆细胞,但需要几天才达到高峰。当同一病原体再次入侵,记忆细胞迅速增殖分化,产生更快更强的二次应答(secondary response),往往在症状出现前就将病原体清除。这种由自身免疫系统产生抗体与记忆细胞的免疫叫主动免疫(active immunity),可通过自然感染获得,也可通过接种疫苗获得。被动免疫(passive immunity)则是直接输入现成抗体,如婴儿从母乳获得的抗体、被蛇咬后注射的抗血清,起效快但持续时间短,因为体内没有产生记忆细胞。群体免疫(herd immunity)指当接种率足够高(麻疹约需 95%)时,病原体难以在人群中传播,从而保护无法接种疫苗的个体。
A vaccine contains inactivated or attenuated pathogens, or their antigens. It triggers a primary immune response that produces plasma cells and memory cells, but only peaks after several days. When the same pathogen enters again, memory cells divide rapidly and produce a faster, larger secondary response that usually clears the pathogen before symptoms appear. Immunity in which your own immune system makes antibodies and memory cells is called active immunity; it can come from natural infection or from vaccination. Passive immunity means receiving ready-made antibodies, such as antibodies passed to a baby in breast milk or antiserum injected after a snake bite; it acts quickly but does not last, because no memory cells are produced. Herd immunity means that when vaccination coverage is high enough (about 95% for measles), the pathogen cannot easily spread through the population, protecting individuals who cannot be vaccinated.
7. 抗生素、抗病毒药与耐药性 Antibiotics, Antivirals and Resistance
抗生素(antibiotics)是杀死或抑制细菌的化学物质,例如青霉素抑制细菌细胞壁的合成。抗生素对病毒无效,因为病毒没有自己的细胞壁和核糖体,而是利用宿主细胞的机制复制,因此病毒病需要用抗病毒药(antivirals),例如抑制 HIV 逆转录酶(reverse transcriptase)的药物,阻止病毒 RNA 逆转录为 DNA。抗生素耐药性(antibiotic resistance)是 OCR 论述题的热点:细菌在繁殖中自发产生基因突变,个别细菌因突变获得抗药性;在抗生素的选择压力(selection pressure)下,敏感菌被杀死,耐药菌存活并大量繁殖,经过自然选择耐药菌比例不断上升,形成耐甲氧西林金黄色葡萄球菌(MRSA)等超级细菌。防控措施包括:医生不随意开抗生素、患者按疗程服完药物、医院严格消毒、研发新药,以及避免在畜牧业中滥用。
Antibiotics are chemicals that kill or inhibit bacteria; for example, penicillin stops bacteria from synthesising their cell walls. Antibiotics do not work against viruses, because viruses lack their own cell walls and ribosomes and replicate using host cell machinery; viral diseases are treated with antivirals, such as drugs that inhibit the HIV reverse transcriptase and stop viral RNA being converted into DNA. Antibiotic resistance is a favourite OCR essay topic: random gene mutations arise as bacteria reproduce, and occasional mutants gain resistance; under the selection pressure of antibiotics, susceptible bacteria are killed while resistant ones survive and multiply, so through natural selection the resistant proportion keeps rising, producing superbugs such as MRSA, methicillin-resistant Staphylococcus aureus. Control measures include avoiding unnecessary prescriptions, completing the full course of treatment, strict hospital hygiene, developing new drugs, and limiting misuse in farming.
8. 典型疾病:结核病与 HIV Tuberculosis and HIV Case Studies
结核病(tuberculosis, TB)由结核分枝杆菌引起,主要通过飞沫传播,感染后细菌在肺部繁殖,引起咳嗽、胸痛、咳血和体重下降;细菌被巨噬细胞吞噬后可在肉芽肿(granuloma)内长期潜伏,免疫力下降时重新激活。诊断依赖胸透与痰检,治疗用多种抗生素联合用药数个月,卡介苗(BCG)提供一定保护。HIV 是逆转录病毒,主要感染辅助性 T 细胞,导致细胞免疫被削弱;随着 T 细胞数量下降,患者进入 AIDS 期,极易感染机会性病原体并发展出罕见肿瘤。HIV 通过性接触、血液和母婴传播,不会通过日常接触或蚊虫叮咬传播;目前无疫苗,但抗逆转录病毒治疗(ART)能把病毒载量压到检测不到的水平,既延长生命又显著降低传播风险。两种病常被组合考查:TB 与 HIV 互相促进,HIV 患者免疫力低下使潜伏的结核菌更容易活化,结核病也成为 AIDS 患者最常见的死因之一。
Tuberculosis is caused by the Mycobacterium tuberculosis bacterium and spreads mainly by droplet infection. The bacteria multiply in the lungs, causing cough, chest pain, coughing blood and weight loss; engulfed bacteria can stay dormant for years inside granulomas and reactivate when immunity drops. Diagnosis relies on chest X-rays and sputum tests; treatment uses a combination of several antibiotics over months, and the BCG vaccine offers some protection. HIV is a retrovirus that mainly infects helper T cells and progressively destroys cell-mediated immunity; as T cell numbers fall, the patient enters the AIDS stage and becomes vulnerable to opportunistic pathogens and rare tumours. HIV spreads through sexual contact, blood and mother-to-child transmission, never through casual contact or mosquito bites. There is no vaccine, but antiretroviral therapy (ART) can suppress the viral load to undetectable levels, prolonging life and greatly reducing transmission. The two diseases are often tested together: HIV and TB reinforce each other, because the weakened immunity of HIV patients allows dormant tuberculosis bacteria to reactivate, and TB is one of the most common causes of death in AIDS patients.
9. OCR 答题要点与高频术语 Command Words and Key Terms
OCR 卷面常见命令词必须分清:describe 只要求陈述所见变化,例如描述感染率曲线先升后平;explain 必须给出原因,答题用 because 连接因果,例如疫苗覆盖率上升导致感染率下降是因为群体免疫使传播链中断;compare 要同时写相同点和不同点;evaluate 则要先给正反两面证据再下结论,例如评价全民接种政策的利弊。名词解释采用"定义加例子"结构:先写该术语的生物学定义,再补一个具体实例,例如抗体是能与特定抗原结合并使其失活的蛋白质,如中和毒素的免疫球蛋白。六分论述题建议用段落分点、术语加粗、必要时配简图,先写机制再写实例再写意义,保证每个得分关键词都出现:pathogen, antigen, antibody, memory cell, secondary response。
You must know the OCR command words. Describe means state what you can see, for example that the infection rate curve rises then levels off. Explain means give reasons, linking cause and effect with because, for example vaccine coverage reduces infection rates because herd immunity breaks the chain of transmission. Compare means write about both similarities and differences. Evaluate means present evidence for and against, then reach a judgement, for example weighing the pros and cons of a mass vaccination programme. For definitions, use the structure definition plus example: state the biological meaning, then add a concrete instance, for example an antibody is a protein that binds to a specific antigen and makes it inactive, such as immunoglobulin neutralising a toxin. For six-mark essays, write in clear paragraphs, use key terms, and add a simple diagram if useful: mechanism first, then an example, then the significance, and make sure every credit keyword appears: pathogen, antigen, antibody, memory cell, secondary response.
小结:疾病与免疫章节的复习主线是"病原体如何进入、人体如何防御、医学如何干预"三层逻辑。每个知识点都能回答三个问题:它是什么(定义)、为什么(机制)、有什么意义(应用,如疫苗、抗生素管理)。把本文的传播途径分类、吞噬五步、T 细胞与 B 细胞分工、主动与被动免疫对比、MRSA 自然选择链条完整复述一遍,再配合 OCR 真题训练,这章的分数就能稳稳拿下。
Summary: revise this chapter along a three-layer logic: how pathogens enter, how the body defends itself, and how medicine intervenes. For every knowledge point you should answer three questions: what is it, what is the mechanism, and why does it matter in applications such as vaccination and antibiotic stewardship. If you can retell the transmission routes, the five steps of phagocytosis, the division of labour between T and B cells, the active-versus-passive immunity comparison and the full MRSA natural selection chain, then practise with real OCR past papers, the marks in this chapter are yours.
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