OCR生物A-Level疾病与免疫系统指南 | 13. OCR Biology A-Level Disease and Immune System Guide

OCR生物A-Level疾病与免疫系统指南

疾病与免疫是OCR A-Level生物（H420）Module 4的核心章节，也是每年必考的得分重点。本指南按照考试大纲系统梳理四大类病原体、传染病的传播途径、人体免疫应答的完整过程，以及疫苗与抗生素背后的考点逻辑，帮助同学们建立清晰的知识框架，在考试中把该拿的分数稳稳拿到手。

Disease and immunity is the core topic of Module 4 in OCR Biology A (H420) and a guaranteed source of marks every year. This guide follows the specification to organise the four types of pathogen, the transmission routes of communicable diseases, the complete immune response, and the exam logic behind vaccines and antibiotics, so that you can build a clear framework and secure every available mark.

一、病原体的四大类型 | Four Types of Pathogen

传染病（communicable disease）由病原体（pathogen）引起，能够在生物个体之间传播。OCR大纲要求掌握四大类病原体：细菌（bacteria）是单细胞原核生物，无核膜，例如引起结核病的结核分枝杆菌（Mycobacterium tuberculosis）；病毒（viruses）没有细胞结构，仅由核酸和蛋白质外壳（capsid）组成，必须寄生在活细胞内利用宿主机制繁殖，如HIV和流感病毒；真菌（fungi）是真核生物，细胞有细胞壁和核膜，如引起马铃薯晚疫病的致病菌；原生生物（protoctists）同样是真核生物，例如引起疟疾的疟原虫（Plasmodium）。掌握每类病原体的结构特点，是解释传播方式和设计防治手段的基础。

A communicable disease is an illness caused by a pathogen that can be transmitted between organisms. In OCR Biology A (H420) Module 4 you need to know four main types of pathogen. Bacteria are single-celled prokaryotes without a nuclear membrane, for example Mycobacterium tuberculosis, which causes tuberculosis. Viruses have no cellular structure; they consist of nucleic acid enclosed in a protein capsid and must replicate inside living host cells using host machinery, as with HIV and influenza. Fungi are eukaryotic organisms with cell walls and a nuclear membrane, such as the pathogen behind potato blight. Protoctists are also eukaryotic, including Plasmodium, the parasite that causes malaria. Understanding the structure of each type is the foundation for explaining transmission and designing control measures.

二、传染病如何传播 | How Communicable Diseases Spread

传播途径是OCR的常考选择题和简答题素材。直接传播包括：直接接触（direct contact），病原体通过皮肤伤口进入人体；性接触（sexual contact），HIV等病原体经黏膜传播；飞沫传播（droplet infection），患者咳嗽、打喷嚏时释放含病原体的飞沫，结核病和流感都以此传播。间接传播包括：空气传播（airborne），飞沫水分蒸发后病原体悬浮在空气中被吸入；经污染的水或食物传播，例如污水中的病原体进入饮水系统；媒介传播（vector transmission），由动物媒介携带传播，例如雌性按蚊（Anopheles mosquito）传播疟原虫。答题时务必准确区分飞沫传播与空气传播，前者需要近距离接触，后者可在空气中存活较久。

Transmission routes appear frequently in multiple-choice and short-answer questions. Direct transmission includes direct contact, where pathogens enter through breaks in the skin; sexual contact, which spreads HIV across mucous membranes; and droplet infection, where coughing or sneezing releases pathogen-containing droplets, the route for tuberculosis and influenza. Indirect transmission includes airborne spread, where dried droplets leave pathogens suspended in the air to be inhaled; transmission through contaminated water or food, for example pathogens from sewage entering the water supply; and vector transmission, where an animal vector carries the pathogen, such as the female Anopheles mosquito spreading Plasmodium. In answers, distinguish droplet infection, which needs close contact, from airborne transmission, where pathogens survive longer in the air.

三、四种代表性疾病实例 | Four Representative Diseases

结核病（tuberculosis, TB）由结核分枝杆菌引起，主要感染肺部，通过飞沫传播，症状包括持续咳嗽、痰中带血、发热和体重下降，可用抗生素治疗。艾滋病（AIDS）由人类免疫缺陷病毒（HIV）引起，HIV专门侵染辅助T淋巴细胞（helper T cells），逐步摧毁免疫系统，通过性接触、血液和母婴途径传播。流感（influenza）由流感病毒引起，损伤呼吸道上皮细胞，由于病毒表面抗原频繁变异，人体难以建立持久免疫。疟疾（malaria）由疟原虫引起，以雌性按蚊为媒介，疟原虫先侵染肝细胞，再进入红细胞繁殖，造成周期性寒战发热。对比记忆这四种疾病，答题时按病原体、传播方式、感染部位、防治手段四要素组织答案。

Tuberculosis is caused by Mycobacterium tuberculosis, mainly infects the lungs, spreads by droplet infection, and produces persistent cough, blood-stained sputum, fever and weight loss; it is treated with antibiotics. AIDS is caused by HIV, which specifically infects helper T lymphocytes and progressively destroys the immune system; it spreads through sexual contact, blood and mother-to-child transmission. Influenza is caused by the flu virus, which damages the epithelial cells of the respiratory tract; because its surface antigens mutate frequently, lasting immunity is hard to establish. Malaria is caused by Plasmodium, transmitted by female Anopheles mosquitoes; the parasite first infects liver cells, then enters red blood cells to reproduce, producing recurrent fevers and chills. Memorise these four diseases side by side and organise answers around pathogen, transmission, site of infection and control.

四、植物病害的代表 | Plant Diseases: TMV and Ring Rot

OCR大纲还要求掌握两种典型植物病害。烟草花叶病毒（tobacco mosaic virus, TMV）感染烟草等多种植物，使叶片出现花叶状斑纹（mottling），叶绿体受损，光合速率下降，植株生长缓慢、产量降低，主要通过汁液接触和农业工具传播。环腐病（ring rot）由细菌引起，侵染马铃薯和番茄，切开块茎可见黑色环状腐烂组织，主要通过受污染的机械、土壤和灌溉水传播。植物病害的防治策略包括：培育和种植抗病品种、实行轮作（crop rotation）、使用杀菌剂、及时清除并销毁病株，以及加强检疫防止病原体传入。

The specification also covers two typical plant diseases. Tobacco mosaic virus infects tobacco and many other plants, producing mottled patches on leaves; damaged chloroplasts lower the rate of photosynthesis, slowing growth and reducing yield. It spreads mainly through sap contact and contaminated tools. Ring rot is caused by a bacterium that infects potatoes and tomatoes; cut tubers show black rings of decaying tissue, and the disease spreads through contaminated machinery, soil and irrigation water. Control measures include growing resistant varieties, crop rotation, applying fungicides, removing and destroying infected plants promptly, and enforcing quarantine to keep pathogens out.

五、人体的第一道防线 | Non-Specific Defences

人体抵御病原体的第一道防线是非特异性免疫（non-specific defence），对所有病原体一视同仁。物理屏障包括：皮肤（skin）作为完整屏障阻止病原体进入，表皮细胞不断脱落带走附着的微生物；呼吸道内壁的黏液（mucus）黏附病原体，纤毛（cilia）以波浪运动将黏液向上推送至咽部，被咳出或咽下。化学屏障包括：胃酸（stomach acid）的强酸性环境杀死随食物进入的细菌；泪液和唾液中的溶菌酶（lysozyme）水解细菌细胞壁；皮肤分泌的皮脂（sebum）含有抑菌物质。此外，炎症反应（inflammatory response）使局部血管扩张、毛细血管通透性增加，白细胞大量聚集到感染部位吞噬病原体。

The first line of defence is non-specific immunity, which acts equally against all pathogens. Physical barriers include the skin, an intact barrier whose epidermal cells are constantly shed, carrying microbes away, and mucus in the airways, which traps pathogens while cilia sweep it upwards to the throat to be coughed out or swallowed. Chemical barriers include stomach acid, whose strongly acidic environment kills bacteria swallowed with food; lysozyme in tears and saliva, which hydrolyses bacterial cell walls; and sebum from the skin, which contains antimicrobial substances. The inflammatory response widens local blood vessels, increases capillary permeability and attracts large numbers of white blood cells to the infected site to engulf pathogens.

六、吞噬作用的五个步骤 | Phagocytosis Step by Step

吞噬作用（phagocytosis）是非特异性免疫的重要环节，由吞噬细胞（phagocytes）完成，也是OCR实验类题目的常客。过程分五步：第一步，吞噬细胞表面的受体识别病原体表面的抗原（antigen）；第二步，细胞膜内陷包裹病原体，形成吞噬小体（phagosome），即吞噬泡；第三步，溶酶体（lysosome）与吞噬小体融合；第四步，溶酶体中的水解酶（lysozymes）将病原体水解消化；第五步，吞噬细胞把病原体碎片呈递到细胞表面，供T淋巴细胞识别，从而把非特异性免疫与特异性免疫衔接起来。考试要求按顺序写全这五步，每步一个得分点。

Phagocytosis is a key part of non-specific immunity carried out by phagocytes, and a favourite for practical-style questions. It proceeds in five steps. First, receptors on the phagocyte surface recognise antigens on the pathogen. Second, the cell membrane invaginates and engulfs the pathogen, forming a phagosome. Third, a lysosome fuses with the phagosome. Fourth, hydrolytic enzymes from the lysosome digest the pathogen. Fifth, the phagocyte presents pathogen fragments on its surface for T lymphocytes to recognise, linking non-specific immunity to the specific response. In exams, write all five steps in order, because each step carries a mark.

七、T淋巴细胞与细胞介导免疫 | Cell-Mediated Immunity

细胞介导免疫（cell-mediated immunity）由T淋巴细胞执行，针对的是已经被病原体感染的体细胞。过程如下：巨噬细胞等抗原呈递细胞（antigen-presenting cell）将抗原片段呈现在细胞表面的MHC分子上；辅助T细胞（helper T cells）表面的受体与抗原结合后被激活，通过有丝分裂大量增殖；一部分子细胞分化为细胞毒性T细胞（cytotoxic T cells），释放穿孔素（perforin）在感染细胞膜上形成孔洞，使细胞裂解死亡，病原体失去藏身之处；另一部分分化为记忆T细胞（memory T cells），在再次感染时迅速大量增殖。注意：整个过程中T细胞不分泌抗体。

Cell-mediated immunity is carried out by T lymphocytes and targets body cells that are already infected. Antigen-presenting cells such as macrophages display antigen fragments on MHC molecules at their surface. Helper T cells whose receptors bind the antigen become activated and divide by mitosis. Some daughter cells become cytotoxic T cells, which release perforin to create pores in the infected cell membrane, causing the cell to lyse and destroying the pathogen's refuge. Others become memory T cells, which proliferate rapidly on re-infection. Note that T cells never secrete antibodies in this process.

八、B淋巴细胞与体液免疫 | Humoral Immunity

体液免疫（humoral immunity）由B淋巴细胞执行，针对的是体液（血液和淋巴）中的病原体。每个B细胞表面带有一种特异性抗体，相当于专一的受体。当某一种抗原与对应B细胞表面的抗体结合，这一过程称为克隆选择（clonal selection），该B细胞被激活并大量增殖，称为克隆扩增（clonal expansion）；一部分子细胞分化为浆细胞（plasma cells），每秒钟可分泌数千个抗体进入体液，中和病原体和毒素；另一部分分化为记忆B细胞（memory B cells），寿命长，能在再次接触抗原时迅速增殖分化。记忆B细胞的存在正是疫苗能够提供长期免疫力的根本原因。

Humoral immunity is carried out by B lymphocytes and targets pathogens in body fluids such as blood and lymph. Each B cell carries one specific antibody on its surface, acting as a unique receptor. When an antigen binds the antibodies on a matching B cell, the process is called clonal selection; the B cell is activated and divides repeatedly in clonal expansion. Some daughter cells become plasma cells, which secrete thousands of antibodies per second into body fluids to neutralise pathogens and toxins. Others become memory B cells, which live for a long time and proliferate rapidly when the same antigen appears again. Memory B cells are the fundamental reason vaccines provide long-lasting immunity.

九、抗体的结构与作用方式 | Antibody Structure and Action

抗体（antibody）是由浆细胞分泌的免疫球蛋白，呈Y形，由四条多肽链组成，属于四级结构（quaternary structure）蛋白质。Y形两臂顶端是可变区（variable region），氨基酸序列因抗体而异，形成与特定抗原形状互补的抗原结合位点（antigen-binding site），决定了抗体的特异性；Y形主干是恒定区（constant region），同一类抗体之间完全相同。抗体通过多种机制消灭病原体：凝集作用（agglutination）使病原体聚集成团，难以扩散且便于吞噬细胞吞噬；中和作用（neutralisation）阻断毒素与受体的结合、阻止病毒进入宿主细胞；调理作用（opsonisation）给病原体贴上标记，促进吞噬细胞识别和吞噬。

Antibodies are immunoglobulins secreted by plasma cells. They are Y-shaped molecules made of four polypeptide chains with a quaternary structure. The tips of the Y arms form the variable region, whose amino acid sequence differs between antibodies and creates an antigen-binding site complementary to a specific antigen, determining the antibody's specificity. The stem forms the constant region, identical within the same class of antibodies. Antibodies act through several mechanisms: agglutination clumps pathogens together so they spread less easily and are more readily engulfed by phagocytes; neutralisation blocks toxins from binding receptors and stops viruses entering host cells; opsonisation tags pathogens so phagocytes recognise and destroy them more efficiently.

十、疫苗与群体免疫 | Vaccines and Herd Immunity

疫苗（vaccine）含有死亡或减毒的病原体、病原体的抗原片段或类毒素，不会真正致病，但能刺激免疫系统产生记忆细胞。当人体再次接触同一种病原体时，记忆细胞迅速增殖分化，产生大量抗体，这一二次免疫应答（secondary response）比初次应答更快、更强，往往在症状出现之前就把病原体清除。群体免疫（herd immunity）的原理是：当人群中接种疫苗的比例足够高，病原体难以在人与人之间传播，那些无法接种的人（如婴儿、老人和免疫缺陷者）也间接得到保护。疫苗的局限性：HIV和流感病毒抗原变异快，疫苗需要不断更新；疫苗储存运输需要冷链，在偏远地区难以保证。

A vaccine contains dead or attenuated pathogens, pathogen antigen fragments or toxoids, which cannot cause disease but stimulate the immune system to produce memory cells. When the body meets the same pathogen again, memory cells proliferate rapidly and produce large numbers of antibodies; this secondary response is faster and stronger than the primary response and usually clears the pathogen before symptoms appear. Herd immunity works because, when a high enough proportion of the population is vaccinated, the pathogen struggles to spread between people, so those who cannot be vaccinated, such as babies, the elderly and the immunocompromised, are indirectly protected. Limitations include the rapid antigen variation of HIV and influenza, which forces constant vaccine updates, and the cold chain required for storage and transport, which is hard to guarantee in remote areas.

十一、抗生素与耐药性 | Antibiotics and Resistance

抗生素（antibiotics）是由微生物产生的天然或半合成物质，能杀死细菌或抑制其生长，作用机制包括抑制细菌细胞壁合成、抑制蛋白质合成等。抗生素对病毒无效：病毒在宿主细胞内利用宿主自身的酶和核糖体进行繁殖，抗生素若针对这些机制就会同时伤害人体细胞。耐药性是OCR的高频考点：细菌DNA发生随机突变（mutation），个别细菌获得耐药性；抗生素杀死了敏感菌，耐药菌存活下来并繁殖，把耐药基因遗传给后代，群体中耐药菌的比例逐渐上升。滥用抗生素、不按疗程服药加速了这一自然选择过程，催生了耐甲氧西林金黄色葡萄球菌（MRSA）等超级细菌。

Antibiotics are natural or semi-synthetic substances produced by microorganisms that kill bacteria or inhibit their growth, for example by blocking cell wall synthesis or protein synthesis. They are ineffective against viruses because viruses replicate inside host cells using the host's own enzymes and ribosomes; targeting those mechanisms would damage human cells too. Antibiotic resistance is a frequent OCR topic: random mutation in bacterial DNA gives a few bacteria resistance; antibiotics kill the susceptible cells, so resistant bacteria survive, reproduce and pass the resistance gene to offspring, gradually increasing its frequency in the population. Overuse of antibiotics and incomplete courses accelerate this natural selection, producing superbugs such as MRSA.

十二、备考建议与常见失分点 | Exam Tips and Common Mistakes

OCR Disease章节的备考要点：第一，精确定义关键术语，如抗原、抗体、吞噬作用、细胞介导免疫，定义题常占2至3分，措辞要严格；第二，能默写细胞介导免疫和体液免疫的完整步骤流程，考试常要求排序或补全流程图；第三，区分易混概念，如非特异性免疫与特异性免疫、细胞介导免疫与体液免疫、主动免疫与被动免疫；第四，准备应用题，例如解释流感疫苗为何每年需要重新接种（抗原变异）、抗生素为何不能治疗流感（病毒无细胞结构，利用宿主机制繁殖）；第五，复习抑菌圈（zone of inhibition）测量实验，掌握无菌操作（aseptic technique）的关键步骤。常见失分点是步骤顺序写错、术语混用以及忘记指出病毒对抗生素不敏感的根本原因。

Key revision tips for the OCR Disease module: first, define key terms precisely, such as antigen, antibody, phagocytosis and cell-mediated immunity, because definition questions carry two to three marks and require exact wording. Second, memorise the full sequences of cell-mediated and humoral immunity, since exams often ask you to order the steps or complete a flow diagram. Third, distinguish easily confused pairs: non-specific versus specific immunity, cell-mediated versus humoral immunity, active versus passive immunity. Fourth, prepare for application questions, such as why flu vaccines need reformulating every year due to antigen variation, or why antibiotics cannot cure influenza, because viruses have no cellular structure and replicate using host machinery. Fifth, revise the zone of inhibition practical, including the key steps of aseptic technique. Common mark losses come from wrong step order, mixed-up terminology and failing to state the fundamental reason why viruses are not affected by antibiotics.

需要专业A-Level辅导？
16621398022
同微信 | 免费咨询课程规划
访问 aleveler.com 了解更多

更多咨询请联系16621398022（同微信）