📚 Immune System: Key Points for A-Level Edexcel Biology | 免疫系统 考点精讲
The immune system is a complex network of cells, tissues, and molecules that defends the body against pathogens, including bacteria, viruses, fungi, and parasites. For Edexcel A-Level Biology, a deep understanding of both non‑specific and specific defences, the roles of B and T lymphocytes, antibody structure and function, as well as immunity, vaccination, and immune disorders, is essential. This revision guide condenses the key concepts and exam‑focused details.
免疫系统是一个由细胞、组织和分子组成的复杂网络，保护身体免受病原体（包括细菌、病毒、真菌和寄生虫）的侵害。对于Edexcel A-Level生物学，深入理解非特异性防御与特异性防御、B细胞和T细胞的作用、抗体结构与功能，以及免疫、疫苗接种和免疫系统疾病至关重要。本复习指南浓缩了核心概念和考试重点。
1. Overview of Defence Mechanisms | 防御机制概述
The body has two main lines of defence against pathogens: non‑specific (innate) defences, which respond rapidly and in the same way to all pathogens, and specific (adaptive) defences, which target particular pathogens and generate immunological memory.
人体对病原体有两条主要防线：非特异性（先天）防御，对所有病原体作出快速且相同的反应；特异性（适应性）防御，针对特定病原体并产生免疫记忆。
Non‑specific defences include physical barriers such as the skin and mucous membranes, chemical defences like lysozyme in tears and stomach acid, and cellular defences involving phagocytes. Specific defences involve lymphocytes—B cells and T cells—and the production of antibodies.
非特异性防御包括物理屏障，如皮肤和粘膜；化学防御，如眼泪中的溶菌酶和胃酸；以及涉及吞噬细胞的细胞防御。特异性防御涉及淋巴细胞——B细胞和T细胞——以及抗体的产生。
2. Non‑Specific Defences | 非特异性防御
The skin acts as a physical barrier; its outer layer of dead, keratinised cells blocks pathogen entry. Sebum and sweat contain antimicrobial substances that lower skin pH, inhibiting microbial growth.
皮肤作为物理屏障；其外层死亡的角质化细胞阻止病原体进入。皮脂和汗液含有抗菌物质，降低皮肤pH值，抑制微生物生长。
Mucous membranes lining the respiratory, digestive, and reproductive tracts trap pathogens in sticky mucus, which is then swept away by cilia or expelled by sneezing and coughing. Hydrochloric acid in the stomach destroys ingested pathogens, while lysozyme in nasal secretions and tears breaks down bacterial cell walls.
呼吸道、消化道和生殖道内的粘膜用黏稠的黏液捕获病原体，然后由纤毛扫走，或通过打喷嚏和咳嗽排出。胃中的盐酸杀死摄入的病原体，而鼻腔分泌物和眼泪中的溶菌酶分解细菌细胞壁。
If pathogens breach these barriers, the inflammatory response is triggered: mast cells release histamine, causing vasodilation and increased capillary permeability. This leads to redness, heat, swelling, and pain, attracting phagocytes to the site of infection.
如果病原体突破这些屏障，会触发炎症反应：肥大细胞释放组胺，引起血管扩张和毛细血管通透性增加。这导致红、热、肿、痛，并吸引吞噬细胞到感染部位。
3. Phagocytosis and Antigen Presentation | 吞噬作用与抗原呈递
Phagocytes (neutrophils and macrophages) engulf pathogens by extending pseudopodia around them, forming a phagosome. The phagosome fuses with a lysosome to form a phagolysosome, where hydrolytic enzymes digest the pathogen.
吞噬细胞（中性粒细胞和巨噬细胞）通过伸出伪足包围病原体，形成吞噬体。吞噬体与溶酶体融合形成吞噬溶酶体，其中的水解酶消化病原体。
After digestion, macrophages present fragments of the pathogen’s antigens on their surface using major histocompatibility complex (MHC) class II molecules. This antigen presentation is crucial for activating helper T cells and initiating the specific immune response.
消化后，巨噬细胞利用主要组织相容性复合体（MHC）II类分子将病原体抗原片段呈递在其表面。这种抗原呈递对于激活辅助T细胞和启动特异性免疫反应至关重要。
4. Cell‑Mediated Immunity: T Lymphocytes | 细胞介导免疫：T淋巴细胞
Cell‑mediated immunity involves T lymphocytes (T cells) and is particularly effective against intracellular pathogens, such as viruses, and abnormal cells, including cancer cells.
细胞介导免疫涉及T淋巴细胞，对胞内病原体（如病毒）以及异常细胞（包括癌细胞）特别有效。
Immature T cells migrate from the bone marrow to the thymus, where they mature and develop specific T cell receptors (TCRs). Helper T cells (TH cells) are activated when their TCR binds to an antigen presented on an MHC class II molecule of an antigen‑presenting cell (APC). Once activated, TH cells secrete cytokines that stimulate B cells, cytotoxic T cells, and macrophages.
未成熟的T细胞从骨髓迁移到胸腺，在那里成熟并形成特异性T细胞受体（TCR）。当辅助T细胞（TH细胞）的TCR与抗原呈递细胞（APC）上MHC II类分子呈递的抗原结合时，TH细胞被激活。激活后，TH细胞分泌细胞因子，刺激B细胞、细胞毒性T细胞和巨噬细胞。
Cytotoxic T cells (TC cells) are activated by antigens presented on MHC class I molecules, which are found on all nucleated cells. TC cells release perforin and granzymes that induce apoptosis in infected host cells.
细胞毒性T细胞（TC细胞）由MHC I类分子呈递的抗原激活，而MHC I类分子存在于所有有核细胞上。TC细胞释放穿孔素和颗粒酶，诱导被感染宿主细胞凋亡。
5. Humoral Immunity: B Lymphocytes | 体液免疫：B淋巴细胞
Humoral immunity involves B lymphocytes (B cells) and targets pathogens in body fluids (humours). B cells mature in the bone marrow and carry specific IgM antibodies on their surface, which act as B cell receptors (BCRs).
体液免疫涉及B淋巴细胞，靶向体液中的病原体。B细胞在骨髓中成熟，并在其表面携带特异性IgM抗体，这些抗体作为B细胞受体（BCR）。
When a B cell encounters its complementary antigen and receives cytokine signals from activated TH cells, it becomes activated. The activated B cell undergoes clonal selection and clonal expansion: it divides rapidly to produce a large population of identical B cells. Most differentiate into plasma cells that secrete huge quantities of antibodies into the blood; some become memory B cells that persist for years, providing long‑term immunity.
当B细胞遇到互补抗原并从激活的TH细胞接收细胞因子信号时，被激活。激活的B细胞经历克隆选择和克隆扩增：迅速分裂产生大量相同的B细胞。大多数分化为浆细胞，向血液中分泌大量抗体；一些成为记忆B细胞，存活多年，提供长期免疫力。
6. Antibody Structure and Function | 抗体结构与功能
Antibodies (immunoglobulins) are Y‑shaped glycoproteins composed of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bonds.
抗体（免疫球蛋白）是Y形糖蛋白，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。
Each antibody has a variable region at the tips of the Y arms, where the amino acid sequence varies greatly between different antibodies. This variable region forms the antigen‑binding site and is specific to a particular antigen. The rest of the molecule is the constant region, which determines the class of antibody (IgM, IgG, IgA, IgE, IgD) and interacts with immune cells.
每个抗体在Y臂尖端有一个可变区，不同抗体间该区域氨基酸序列差异很大。这个可变区形成抗原结合位点，对特定抗原具有特异性。分子的其余部分是恒定区，决定抗体的类别（IgM、IgG、IgA、IgE、IgD），并与免疫细胞相互作用。
Antibodies exert their effects by neutralising toxins and preventing pathogen entry into cells, agglutinating pathogens to make them easier for phagocytes to engulf, and marking pathogens for destruction (opsonisation). They can also activate the complement system, leading to lysis of bacterial cells.
抗体通过中和毒素和阻止病原体进入细胞、凝集病原体使其更容易被吞噬细胞吞噬、以及标记病原体以供破坏（调理作用）来发挥作用。它们还能激活补体系统，导致细菌细胞裂解。
7. Active and Passive Immunity | 主动免疫与被动免疫
| Feature | Active Immunity | Passive Immunity |
|---|---|---|
| Production of antibodies | Made by the individual’s own B cells | Received from an external source |
| Onset | Slow (days to weeks) | Immediate |
| Duration | Long‑lasting (memory cells formed) | Short‑lived (no memory cells formed) |
| Natural example | Infection with a pathogen | Antibodies from mother to baby via placenta/breast milk |
| Artificial example | Vaccination | Injection of antiserum (e.g., tetanus immunoglobulin) |
Active immunity results when the body is exposed to an antigen and generates its own immune response, including memory cells. Passive immunity involves the transfer of ready‑made antibodies, providing immediate but temporary protection without immunological memory.
当身体接触抗原并产生自身免疫应答（包括记忆细胞）时，即形成主动免疫。被动免疫涉及预制成抗体的转移，提供即时但暂时的保护，不形成免疫记忆。
8. Vaccination and Herd Immunity | 疫苗接种与群体免疫
Vaccines contain antigens derived from a pathogen, which may be inactivated or attenuated whole organisms, isolated proteins, or genetically engineered subunits. The antigens trigger a primary immune response, producing memory B and T cells without causing disease.
疫苗含有源自病原体的抗原，可以是灭活或减毒的全微生物、分离的蛋白质或基因工程亚单位。这些抗原触发初次免疫应答，产生记忆B和T细胞，但不引起疾病。
Upon subsequent exposure to the actual pathogen, the secondary immune response is rapid and strong, preventing illness. Vaccination programmes can lead to herd immunity: if a high percentage of a population is immunised, the chain of transmission is broken, protecting unvaccinated individuals.
当随后接触真正的病原体时，二次免疫应答快速而强大，从而防止疾病。疫苗接种项目可达成群体免疫：如果人群中高比例接种，传播链被打破，未接种疫苗的个体也得到保护。
Concerns over vaccine safety and side effects must be weighed against the enormous benefits in controlling infectious diseases. Reliable epidemiological data demonstrate that vaccines have drastically reduced the incidence of diseases such as polio, measles, and smallpox.
对疫苗安全性和副作用的担忧必须与控制传染病的巨大益处进行权衡。可靠的流行病学数据表明，疫苗大大降低了脊髓灰质炎、麻疹和天花等疾病的发病率。
9. Monoclonal Antibodies | 单克隆抗体
Monoclonal antibodies are identical antibodies produced by a single clone of B cells. They are created using the hybridoma technique: a mouse is immunised with the target antigen, its B cells are harvested, and they are fused with myeloma (cancer) cells to form hybridoma cells that divide indefinitely and secrete the specific antibody.
单克隆抗体是由单一B细胞克隆产生的相同抗体。通过杂交瘤技术制备：用目标抗原免疫小鼠，收获其B细胞，与骨髓瘤（癌）细胞融合，形成能无限分裂并分泌特异性抗体的杂交瘤细胞。
Monoclonal antibodies have a wide range of medical applications, including pregnancy testing (detecting hCG), cancer therapy (targeting tumour markers), diagnosis of diseases, and as therapeutic agents for autoimmune conditions. However, ethical considerations regarding animal use and potential side effects must be considered.
单克隆抗体有广泛的医学应用，包括验孕（检测hCG）、癌症治疗（靶向肿瘤标志物）、疾病诊断，以及作为自身免疫病的治疗剂。然而，必须考虑动物使用的伦理问题和潜在副作用。
10. Primary and Secondary Immune Responses | 初次与二次免疫应答
The primary immune response occurs when the immune system first encounters a novel antigen. There is a lag phase of several days before antibodies appear in the blood; the antibody concentration rises slowly, peaks at a relatively low level, and then declines. IgM is typically the first class of antibody produced.
初次免疫应答发生在免疫系统首次接触新抗原时。抗体出现在血液中之前有几天的滞后期；抗体浓度上升缓慢，峰值较低，然后下降。通常最先产生的是IgM类抗体。
In the secondary immune response, the same antigen is encountered again. Memory B and T cells are rapidly activated, leading to a much shorter lag, a much higher antibody concentration (mostly IgG), and prolonged antibody persistence. This rapid response prevents infection from establishing, often before any symptoms appear.
在二次免疫应答中，同一抗原再次遇到。记忆B和T细胞被迅速激活，导致滞后期更短，抗体浓度高得多（主要是IgG），且抗体维持时间更长。这种快速应答在症状出现之前就阻止了感染的建立。
11. Immune System Disorders: Allergies and Autoimmunity | 免疫系统疾病：过敏与自身免疫
An allergy is a hypersensitive immune response to a normally harmless environmental substance (allergen), such as pollen or peanuts. Upon first exposure, plasma cells produce IgE antibodies, which bind to mast cells. On subsequent exposure, the allergen cross‑links the IgE, triggering mast cells to release histamine and other inflammatory mediators, causing symptoms ranging from hay fever to life‑threatening anaphylaxis.
过敏是对通常无害的环境物质（过敏原），如花粉或花生，产生的超敏免疫反应。首次暴露时，浆细胞产生IgE抗体，这些抗体结合在肥大细胞上。再次暴露时，过敏原交联IgE，触发肥大细胞释放组胺和其他炎症介质，引起从花粉症到危及生命的过敏反应等症状。
Autoimmune diseases arise when the immune system fails to distinguish self from non‑self and attacks the body’s own tissues. Examples include type 1 diabetes, where TC cells destroy insulin‑producing beta cells in the pancreas, and rheumatoid arthritis, which involves antibodies against joint tissues. The causes involve genetic predisposition and environmental triggers.
当免疫系统无法区分自身与非自身，并攻击身体自身组织时，就会发生自身免疫病。例如，1型糖尿病中，TC细胞破坏胰腺中产生胰岛素的β细胞；类风湿关节炎则涉及针对关节组织的抗体。病因涉及遗传易感性和环境触发因素。
12. HIV and the Immune System | HIV与免疫系统
Human immunodeficiency virus (HIV) is a retrovirus that primarily infects helper T cells (TH cells), macrophages, and dendritic cells. It binds to the CD4 receptor and a co‑receptor on the cell surface, enters the cell, and uses reverse transcriptase to convert its RNA into DNA, which integrates into the host genome.
人类免疫缺陷病毒（HIV）是一种逆转录病毒，主要感染辅助T细胞（TH细胞）、巨噬细胞和树突状细胞。它与细胞表面的CD4受体和共受体结合，进入细胞，利用逆转录酶将其RNA转化为DNA，并整合到宿主基因组中。
As HIV replicates, it progressively destroys TH cells, weakening the immune system. When the TH cell count falls below a critical level, the body becomes susceptible to opportunistic infections and cancers, leading to acquired immunodeficiency syndrome (AIDS). There is no definitive cure, but antiretroviral drugs can control viral replication and prolong life.
随着HIV复制，它会逐渐破坏TH细胞，削弱免疫系统。当TH细胞计数降至临界水平以下时，身体容易受到机会性感染和癌症的侵袭，导致获得性免疫缺陷综合征（艾滋病）。目前尚无根治方法，但抗逆转录病毒药物能够控制病毒复制并延长生命。
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