A-Level生物 免疫系统 特异非特异防御

A-Level生物 免疫系统 特异非特异防御

免疫系统概述 Overview of the Immune System

免疫系统是人体抵御病原体入侵的复杂防御网络，由多种细胞、组织和器官协同工作，识别并清除外来入侵者，同时避免攻击自身细胞。A-Level大纲要求理解免疫系统如何通过非特异性（先天）和特异性（适应性）两种机制来保护身体。

The immune system is the body’s complex defence network against invading pathogens such as bacteria, viruses, fungi, and parasites. It consists of diverse cells, tissues, and organs working together to recognise and eliminate foreign invaders while avoiding attacks on the body’s own cells. The A-Level Biology syllabus requires understanding how the immune system protects the body through two complementary mechanisms: non-specific (innate) and specific (adaptive) defences.

非特异性防御：第一道防线 Non-Specific Defences: The First Line of Defence

非特异性防御机制对所有病原体一视同仁，反应迅速且不区分入侵者类型。第一道防线是物理和化学屏障。皮肤作为最大的物理屏障，其角质层由死细胞和角蛋白构成，绝大多数病原体无法穿透。皮脂腺分泌皮脂，含脂肪酸降低pH值，抑制微生物生长。

Non-specific defence mechanisms treat all pathogens equally, responding rapidly without distinguishing between invader types. The first line of defence consists of physical and chemical barriers. The skin serves as the largest physical barrier, with its outer keratinised layer composed of dead cells and keratin that most pathogens cannot penetrate. Sebaceous glands on the skin secrete sebum containing fatty acids that lower pH and inhibit microbial growth.

粘膜是另一重要物理屏障。呼吸道、消化道和泌尿生殖道的粘膜分泌粘液，捕获病原体。呼吸道中的纤毛上皮通过有节奏的摆动，将含病原体的粘液推向咽喉，通过咳嗽或吞咽排出。溶菌酶存在于眼泪、唾液和粘液中，水解细菌细胞壁的肽聚糖，导致细菌裂解。

Mucous membranes form another critical physical barrier. The epithelial linings of the respiratory, digestive, and urogenital tracts secrete mucus that traps pathogens. Ciliated epithelium in the respiratory tract sweeps pathogen-laden mucus towards the throat, where it is expelled through coughing or swallowing. Lysozyme, found in tears, saliva, and mucus, hydrolyses peptidoglycan in bacterial cell walls, causing bacterial lysis.

胃中的盐酸创造极酸环境（pH约1.5-2.0），绝大多数随食物进入的微生物在此被杀死。血液中的凝血机制迅速形成血凝块封闭伤口，阻止病原体进入循环系统。

Hydrochloric acid in the stomach creates a highly acidic environment (pH ~1.5-2.0), killing most microorganisms ingested with food and water. Blood clotting rapidly seals wounds, preventing pathogen entry into the circulatory system.

非特异性防御：第二道防线 Non-Specific Defences: The Second Line of Defence

当病原体突破第一道防线后，先天免疫系统的第二道防线立即启动。炎症反应是核心机制：受损组织释放组胺和细胞因子等化学信号，引起局部血管扩张（血管舒张），增加血流量和毛细血管通透性。这导致典型的炎症症状：红、肿、热、痛。增加的血流量带来更多吞噬细胞和血浆蛋白到感染部位。

When pathogens breach the first line of defence, the second line of innate immunity activates immediately. The inflammatory response is the central mechanism: damaged tissue releases chemical signals such as histamine and cytokines, causing local vasodilation, increased blood flow, and heightened capillary permeability. This produces the classic signs of inflammation: redness, swelling, heat, and pain. The increased blood flow delivers more phagocytes and plasma proteins to the infection site.

吞噬作用是先天免疫中最重要的细胞反应。中性粒细胞是最先到达感染部位的吞噬细胞，它们通过识别病原体表面的化学标记物进行趋化运动。到达后，中性粒细胞通过吞噬作用包裹并消化病原体：其细胞膜伸出伪足包围病原体形成吞噬体，随后与溶酶体融合形成吞噬溶酶体，溶酶体中的水解酶和活性氧物质破坏并消化病原体。

Phagocytosis is the most important cellular response in innate immunity. Neutrophils arrive first at infection sites via chemotaxis, then engulf pathogens through phagocytosis: their membrane extends pseudopodia around the pathogen, forming a phagosome that fuses with lysosomes to create a phagolysosome. Hydrolytic enzymes and reactive oxygen species destroy the pathogen.

巨噬细胞是更大的吞噬细胞，由离开血液循环进入组织的单核细胞分化而成。巨噬细胞不仅吞噬病原体，还在抗原呈递中发挥关键作用：它们将处理后的抗原片段展示在MHC II类分子上，为激活特异性免疫做准备。自然杀伤（NK）细胞识别并杀死被病毒感染的细胞和肿瘤细胞，通过释放穿孔素和颗粒酶诱导靶细胞凋亡。

Macrophages are larger phagocytes derived from monocytes entering tissues. They engulf pathogens and play a crucial role in antigen presentation: displaying processed antigen fragments on MHC class II molecules to activate the specific immune response. Natural killer (NK) cells kill virus-infected and tumour cells by releasing perforin and granzymes that induce apoptosis.

特异性免疫应答概述 Overview of the Specific Immune Response

当先天免疫无法清除感染时，适应性免疫系统被激活。与先天免疫不同，特异性免疫具有高度特异性（每种淋巴细胞识别一种特定抗原）、记忆性（再次遇到同一抗原时反应更快更强）和自我耐受性（不攻击自身组织）。特异性免疫主要由B淋巴细胞和T淋巴细胞介导。

When innate immunity fails to clear an infection, the adaptive immune system is activated. Unlike innate immunity, specific immunity is highly specific (each lymphocyte recognises only one particular antigen), possesses memory (subsequent encounters with the same antigen provoke faster and stronger responses), and displays self-tolerance (normally does not attack the body’s own tissues). Specific immunity is primarily mediated by two classes of lymphocytes: B lymphocytes and T lymphocytes.

B细胞在骨髓中成熟，负责体液免疫：产生抗体中和细胞外病原体和毒素。T细胞在胸腺中成熟，负责细胞介导免疫：攻击被感染细胞或通过细胞因子协调免疫反应。每个未成熟淋巴细胞表面都有独特的受体，能识别特定的抗原。

B cells mature in the bone marrow and are responsible for humoral immunity: producing antibodies against extracellular pathogens and toxins. T cells migrate to the thymus for maturation and are responsible for cell-mediated immunity: attacking infected cells or coordinating responses through cytokines. Each immature lymphocyte has unique surface receptors that recognise a specific antigen.

在发育过程中，B细胞和T细胞经历克隆选择。那些能识别自身抗原的淋巴细胞被消除（克隆删除）或失活，只有识别非自身抗原的淋巴细胞得以存活并成熟。成熟的naive淋巴细胞在血液和淋巴器官中循环，等待遇到其特异性抗原。

During development, B and T cells undergo clonal selection: those recognising self-antigens are eliminated or inactivated, while those recognising non-self antigens survive. Mature naive lymphocytes circulate through blood and lymphoid organs, awaiting encounter with their specific antigen.

体液免疫应答 The Humoral Immune Response

体液免疫应答针对细胞外病原体和游离毒素。当naive B细胞通过表面IgM受体遇到互补抗原时，抗原-受体结合触发B细胞激活。大多数B细胞需要辅助T细胞的第二信号才能完全激活。活化的B细胞通过克隆扩增产生大量相同的克隆。

The humoral immune response targets extracellular pathogens and free toxins. When a naive B cell encounters its complementary antigen via surface IgM receptors, antigen-receptor binding triggers B cell activation. Most B cells require a second signal from helper T cells for full activation. Activated B cells undergo clonal expansion, producing large numbers of identical clones.

克隆扩增后，B细胞分化为两类细胞：浆细胞和记忆B细胞。浆细胞是抗体工厂：每个浆细胞每秒可分泌多达2000个相同特异性的抗体分子。它们寿命较短（数天到数周），但在此期间大量产生抗体。抗体通过多种机制中和病原体：凝集（使病原体聚集）、调理作用（标记病原体以促进吞噬）、中和毒素和激活补体系统。

After clonal expansion, B cells differentiate into plasma cells and memory B cells. Plasma cells are antibody factories, secreting up to 2000 identical antibody molecules per second. They are short-lived (days to weeks) but produce large quantities of antibodies. Antibodies neutralise pathogens through agglutination (clumping), opsonisation (tagging for phagocytosis), neutralising toxins, and activating the complement system.

记忆B细胞寿命极长（数年甚至终生），不分泌抗体但保持抗原识别能力。当同一病原体再次入侵时，记忆B细胞迅速分裂为浆细胞，在数小时内产生大量高亲和力抗体。这是疫苗接种的免疫学基础。

Memory B cells are extremely long-lived (years or a lifetime). They do not secrete antibodies but retain antigen recognition. When the same pathogen invades again, memory B cells rapidly divide into plasma cells, producing high-affinity antibodies within hours. This is the immunological basis of vaccination.

抗体结构 Antibody Structure

抗体（免疫球蛋白）是Y形糖蛋白分子，由四条多肽链组成：两条相同的重链和两条相同的轻链，通过二硫键连接。每条链包含恒定区和可变区。可变区位于Y形分子的双臂末端，形成抗原结合位点，其氨基酸序列在不同抗体间高度可变，决定了抗体的特异性。恒定区决定抗体的类别（IgM、IgG、IgA、IgE、IgD），并介导效应功能。

Antibodies (immunoglobulins) are Y-shaped glycoprotein molecules composed of four polypeptide chains: two identical heavy chains and two identical light chains, linked by disulfide bonds. Each chain contains constant and variable regions. The variable regions at the tips of the Y arms form the antigen-binding sites; their amino acid sequences are highly variable between antibodies, determining specificity. The constant regions determine antibody class (IgM, IgG, IgA, IgE, IgD) and mediate effector functions.

在A-Level考试中，你需要了解：IgM是初次应答中最先产生的抗体；IgG是血液中最丰富的抗体，可穿过胎盘提供被动免疫；IgA存在于粘膜分泌物（唾液、眼泪、母乳）中；IgE参与过敏反应和抗寄生虫防御。

For A-Level examinations, key antibody functions: IgM is the first antibody in primary responses; IgG is the most abundant and crosses the placenta for passive fetal immunity; IgA is found in mucosal secretions (saliva, tears, breast milk); IgE is involved in allergic reactions and anti-parasitic defence.

细胞介导免疫应答 The Cell-Mediated Immune Response

细胞介导免疫主要针对细胞内病原体，如病毒和被感染的细胞。这一过程依赖于T淋巴细胞。抗原呈递细胞（APC），如树突状细胞和巨噬细胞，吞噬病原体后将抗原片段展示在MHC II类分子上，呈递给naive辅助T细胞（CD4+ T细胞）。同时，所有有核细胞可以将内源性抗原（如病毒蛋白）展示在MHC I类分子上，供细胞毒性T细胞（CD8+ T细胞）识别。

Cell-mediated immunity primarily targets intracellular pathogens such as viruses and infected cells, depending on T lymphocytes. Antigen-presenting cells (APCs), such as dendritic cells and macrophages, phagocytose pathogens and display antigen fragments on MHC class II molecules, presenting them to naive helper T cells (CD4+). All nucleated cells can also display endogenous antigens (such as viral proteins) on MHC class I molecules, for recognition by cytotoxic T cells (CD8+).

辅助T细胞（Th细胞）是免疫系统的总指挥。一旦被APC激活，Th细胞分泌多种细胞因子（如白细胞介素-2），刺激B细胞增殖和分化、激活细胞毒性T细胞、增强巨噬细胞的吞噬活性。HIV病毒正是因为摧毁CD4+ T细胞而导致免疫缺陷，使患者易受机会性感染。

Helper T cells (Th cells) are the conductors of the immune system. Once activated by an APC, Th cells secrete cytokines (such as interleukin-2) that stimulate B cell proliferation, activate cytotoxic T cells, and enhance macrophage phagocytic activity. HIV causes immunodeficiency by destroying CD4+ T cells, leaving patients vulnerable to opportunistic infections.

细胞毒性T细胞（Tc细胞）是免疫系统的杀手。它们识别展示在MHC I类分子上的非自身抗原，然后释放穿孔素（在靶细胞膜上形成孔道）和颗粒酶（通过孔道进入，激活caspase级联反应诱导凋亡）。细胞毒性T细胞逐个杀死被感染的细胞而不伤害邻近的健康细胞。

Cytotoxic T cells (Tc cells) are the killers of the immune system. They recognise non-self antigens on MHC class I molecules, then release perforin (forming membrane pores) and granzymes (activating caspase cascades to induce apoptosis). They kill infected cells one by one without damaging neighbouring healthy cells.

初次与再次免疫应答 Primary vs Secondary Immune Response

初次免疫应答发生在机体首次接触特定抗原时。潜伏期较长（通常5-10天），因为需要时间进行克隆选择和扩增。抗体浓度上升缓慢，先产生IgM后发生类别转换为IgG。症状通常在抗体达到有效水平前出现。初次应答后，记忆细胞库被建立。

The primary immune response occurs on first encounter with a specific antigen. The lag phase is long (5-10 days) because clonal selection and expansion take time. Antibody concentration rises slowly, with IgM produced first before class switching to IgG. Symptoms usually appear before antibodies reach effective levels. After the primary response, a memory cell pool is established.

再次免疫应答在再次接触同一抗原时发生。潜伏期极短（1-2天），因为记忆细胞已经被准备好。抗体浓度迅速飙升，以IgG为主，亲和力比初次应答更高（亲和力成熟）。通常病原体在引起症状之前就被清除了，因此个体可能根本不知道自己被再次感染了。A-Level考试中经常要求你绘制并比较初次和再次应答的抗体浓度-时间曲线。

The secondary immune response occurs upon re-exposure to the same antigen. The lag phase is extremely short (1-2 days) because memory cells are already primed. Antibody concentration surges rapidly, predominantly IgG, with higher affinity than in the primary response. Pathogens are typically cleared before symptoms develop. A-Level examinations frequently require drawing and comparing antibody concentration-time graphs for primary and secondary responses.

疫苗接种与免疫类型 Vaccination and Types of Immunity

疫苗接种通过模拟初次感染建立免疫记忆而不引起疾病。疫苗含有减毒活病原体、灭活病原体、病原体片段或mRNA编码的病原体抗原。接种后，免疫系统产生初次应答并建立记忆细胞库。当真正病原体入侵时，再次免疫应答迅速启动，预防疾病。

Vaccination establishes immunological memory by simulating a primary infection without causing disease. Vaccines contain live attenuated pathogens, inactivated pathogens, pathogen fragments, or mRNA encoding pathogen antigens. After vaccination, the immune system mounts a primary response and establishes a memory cell pool. When the real pathogen invades, the secondary response activates rapidly, preventing disease.

免疫可分为几种类型。主动自然免疫：因自然感染而获得（如患水痘后终生免疫）。主动人工免疫：通过疫苗接种获得。被动自然免疫：母体IgG穿过胎盘传给胎儿，或母乳IgA传给婴儿。被动人工免疫：注射从他人或动物来源制备的抗体（如蛇毒抗血清）。被动免疫提供即时保护但无记忆细胞形成，保护短暂。

Immunity can be classified into several types. Active natural immunity: acquired through natural infection (e.g., lifelong immunity after chickenpox). Active artificial immunity: acquired through vaccination. Passive natural immunity: maternal IgG crossing the placenta to the fetus, or IgA in breast milk. Passive artificial immunity: injection of antibodies from human or animal sources (e.g., snake antivenom). Passive immunity provides immediate protection but no memory cell formation, so protection is short-lived.

群体免疫是一个关键的流行病学概念：当人群中足够高比例的人具有免疫力时（通过接种或感染），病原体的传播链被打断，即使未接种的个体也受到间接保护。群体免疫阈值取决于病原体的基本传染数(R0)：R0越高，所需接种覆盖率越高。例如，麻疹R0约为12-18，需要约95%的覆盖率才能实现群体免疫。

Herd immunity is a key epidemiological concept: when a sufficiently high proportion of the population is immune (through vaccination or prior infection), transmission chains are interrupted, indirectly protecting even unvaccinated individuals. The herd immunity threshold depends on the pathogen’s basic reproduction number (R0): the higher the R0, the greater the coverage required. For measles (R0 ~12-18), about 95% coverage is needed.

自身免疫与超敏反应 Autoimmunity and Hypersensitivity

当自我耐受失败时，免疫系统攻击自身组织，导致自身免疫病。例如：1型糖尿病中T细胞破坏胰岛β细胞；类风湿性关节炎中免疫复合物沉积在关节；多发性硬化症中免疫系统攻击髓鞘。这些疾病展示了免疫调节失败的后果。

When self-tolerance fails, the immune system attacks the body’s own tissues, causing autoimmune diseases. Examples include: Type 1 diabetes where T cells destroy pancreatic beta cells; rheumatoid arthritis where immune complexes deposit in joints; and multiple sclerosis where the immune system attacks the myelin sheath. These illustrate the consequences of immune regulation failure.

超敏反应是免疫系统对无害抗原的过度反应。I型超敏反应（即时型）由IgE介导：肥大细胞脱颗粒释放组胺，引起过敏性鼻炎、哮喘或严重情况下的过敏性休克。A-Level考试通常涉及过敏反应的机制和肾上腺素作为紧急治疗的作用原理。

Hypersensitivity refers to excessive immune responses against harmless antigens. Type I (immediate) hypersensitivity is IgE-mediated: mast cell degranulation releases histamine, causing allergic rhinitis, asthma, or anaphylactic shock. A-Level examinations commonly address the mechanism of allergic reactions and the role of adrenaline as emergency treatment.

考纲关键要点 Exam Key Points

爱德思(Edexcel)和剑桥(CAIE)考试委员会要求你能够：描述吞噬作用的过程和重要性；解释T和B淋巴细胞在特异性免疫中的不同作用；比较初次和再次免疫应答（包括抗体浓度曲线）；区分主动与被动免疫、天然与人工免疫；解释疫苗接种的保护机制（包括群体免疫）；讨论抗生素与疫苗在防治感染中的区别。

Both Edexcel and CAIE examination boards require you to: describe the process and importance of phagocytosis; explain the different roles of T and B lymphocytes in the specific immune response; compare primary and secondary immune responses (including antibody concentration curves); distinguish between active and passive immunity, natural and artificial immunity; explain how vaccination provides protection (including herd immunity); and discuss the differences between using antibiotics to treat bacterial infections versus vaccines to prevent infections.

典型考题包括：标注抗体结构图；解释疫苗接种为何导致再次免疫应答而非初次应答；从实验数据推断某人是初次还是再次感染；批判性评价MMR疫苗安全性证据。确保能将免疫学概念与疾病传播控制联系起来。

Typical exam question formats include: labelling an antibody structure diagram; explaining why vaccination leads to a secondary rather than primary immune response; deducing from experimental data whether an individual is experiencing primary or secondary infection; and critically evaluating MMR vaccine safety evidence. Ensure you can connect immunological concepts to disease transmission and control.

关键双语术语 Key Bilingual Terms

• Immune system / 免疫系统
• Pathogen / 病原体
• Non-specific defence / 非特异性防御
• Specific immune response / 特异性免疫应答
• Phagocytosis / 吞噬作用
• Inflammation / 炎症反应
• Antigen / 抗原
• Antibody / 抗体
• Immunoglobulin / 免疫球蛋白
• Lymphocyte / 淋巴细胞
• B cell / B细胞
• T cell / T细胞
• Plasma cell / 浆细胞
• Memory cell / 记忆细胞
• Helper T cell / 辅助T细胞
• Cytotoxic T cell / 细胞毒性T细胞
• Antigen-presenting cell / 抗原呈递细胞
• MHC molecule / MHC分子
• Clonal selection / 克隆选择
• Clonal expansion / 克隆扩增
• Primary response / 初次应答
• Secondary response / 再次应答
• Vaccination / 疫苗接种
• Active immunity / 主动免疫
• Passive immunity / 被动免疫
• Herd immunity / 群体免疫
• Autoimmunity / 自身免疫
• Hypersensitivity / 超敏反应