📚 A-Level WJEC Biology: Immune System Exam Focus | A-Level WJEC 生物：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 such as bacteria, viruses, fungi and parasites. For WJEC A-Level Biology, you need to understand the distinction between non‑specific (innate) and specific (adaptive) defences, the roles of B and T lymphocytes, the structure and function of antibodies, and how immunity can be achieved naturally or artificially. This article breaks down every key topic with clear bilingual explanations, ensuring you can tackle exam questions confidently.
免疫系统是一个由细胞、组织和分子组成的复杂网络，负责抵御细菌、病毒、真菌和寄生虫等病原体。在 WJEC A-Level 生物考试中，你需要理解非特异性（先天）防御和特异性（适应性）防御的区别、B 淋巴细胞和 T 淋巴细胞的作用、抗体的结构与功能，以及如何通过自然或人工方式获得免疫。本文以清晰的双语解释逐一剖析每个关键主题，确保你能够自信应对考试题目。
1. Overview of Immune Defences | 免疫防御概述
The body uses two major lines of defence against infection: non‑specific (innate) defences that respond instantly to any pathogen, and specific (adaptive) defences that target particular pathogens and create immunological memory. Non‑specific defences include physical barriers like the skin, chemical barriers like stomach acid, and cellular responses such as phagocytosis. Specific defences rely on lymphocytes and the production of antibodies.
人体使用两条主要防线抵御感染：非特异性（先天）防御能够对任何病原体立即作出反应，而特异性（适应性）防御则针对特定病原体并产生免疫记忆。非特异性防御包括皮肤等物理屏障、胃酸等化学屏障以及吞噬作用等细胞反应。特异性防御依赖于淋巴细胞和抗体的产生。
2. Non‑specific Defences: Physical and Chemical Barriers | 非特异性防御：物理与化学屏障
The skin is a tough, waterproof barrier made of keratinised cells that block pathogen entry. Mucous membranes in the airways trap microorganisms, and cilia sweep the mucus towards the throat to be swallowed. Lysozyme, an enzyme found in tears, saliva and nasal secretions, destroys bacterial cell walls. The acidic environment of the stomach (pH around 2) denatures proteins and kills most ingested pathogens.
皮肤是由角质化细胞构成的坚韧防水屏障，可阻挡病原体进入。呼吸道中的粘膜能粘住微生物，纤毛将黏液推向咽喉被吞下。眼泪、唾液和鼻分泌物中含有的溶菌酶能破坏细菌的细胞壁。胃部的酸性环境（pH 约 2）使蛋白质变性，杀死大多数吞入的病原体。
3. Non‑specific Defences: Phagocytosis and Inflammation | 非特异性防御：吞噬作用与炎症
Phagocytes, such as neutrophils and macrophages, recognise foreign particles and engulf them through endocytosis, forming a phagosome. Lysosomes fuse with the phagosome and release hydrolytic enzymes that digest the pathogen. Inflammation is triggered by damaged tissues releasing histamine, which causes vasodilation and increased capillary permeability. This results in redness, heat, swelling and the recruitment of more immune cells to the site.
吞噬细胞（如中性粒细胞和巨噬细胞）识别外来颗粒，通过内吞作用将其吞入，形成吞噬体。溶酶体与吞噬体融合，释放水解酶消化病原体。炎症由受损组织释放组胺引发，组胺引起血管舒张和毛细血管通透性增加，导致红肿、发热、肿胀并招募更多免疫细胞到达该部位。
4. Antigens and Self/Non‑self Recognition | 抗原与自身/非自身识别
An antigen is any molecule that the immune system recognises as foreign and which triggers an immune response. Usually, antigens are proteins or glycoproteins on the surface of pathogens, pollen or transplanted tissues. The body’s own cells carry major histocompatibility complex (MHC) molecules that mark them as ‘self’. Lymphocytes are programmed not to attack self‑cells, but when this system fails, autoimmune disease can occur.
抗原是指任何被免疫系统识别为外来物质并引发免疫反应的分子。抗原通常是病原体、花粉或移植组织表面的蛋白质或糖蛋白。人体自身细胞带有主要组织相容性复合体（MHC）分子，标记为“自身”。淋巴细胞被程序设定为不攻击自身细胞，但当这一系统失效时，就可能发生自身免疫疾病。
5. Specific Immune Response: Role of Lymphocytes | 特异性免疫反应：淋巴细胞的作用
The adaptive immune system involves B lymphocytes (B cells) that mature in the bone marrow, and T lymphocytes (T cells) that mature in the thymus. Each lymphocyte carries a unique receptor that binds to a specific antigen. When a lymphocyte encounters its complementary antigen, it is activated, undergoes clonal selection and proliferates into many identical cells through clonal expansion. Some become effector cells that fight the current infection, while others become long‑lived memory cells.
适应性免疫系统涉及在骨髓中成熟的 B 淋巴细胞（B 细胞）和在胸腺中成熟的 T 淋巴细胞（T 细胞）。每个淋巴细胞携带一种独特的受体，能与特定的抗原结合。当淋巴细胞遇到其互补抗原时被激活，经历克隆选择，并通过克隆扩增增殖为许多相同的细胞。其中一部分成为效应细胞，对抗当前感染，另一部分则成为长寿的记忆细胞。
6. Cell‑mediated Immunity: T Lymphocytes | 细胞介导免疫：T 淋巴细胞
Cell‑mediated immunity involves T cells and is particularly effective against intracellular pathogens, such as viruses, and against cancer cells. Antigen‑presenting cells (APCs) like macrophages display processed antigen fragments on MHC class II molecules. Helper T cells (TH cells) with complementary receptors bind to the antigen‑MHC complex and become activated. Activated TH cells secrete cytokines that stimulate B cells, cytotoxic T cells and macrophages. Cytotoxic T cells (TC cells) recognise infected body cells displaying viral antigens on MHC class I molecules and release perforin, which creates pores in the target cell membrane, leading to apoptosis.
细胞介导免疫涉及 T 细胞，对细胞内病原体（如病毒）和癌细胞特别有效。抗原呈递细胞（APCs），如巨噬细胞，将处理过的抗原片段展示在 MHC II 类分子上。具有互补受体的辅助 T 细胞（TH 细胞）与抗原‑MHC 复合物结合并被激活。活化的 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。细胞毒性 T 细胞（TC 细胞）识别展示病毒抗原的感染体细胞（MHC I 类分子），释放穿孔素，在靶细胞膜上形成孔洞，导致细胞凋亡。
7. Humoral Immunity: B Lymphocytes and Antibodies | 体液免疫：B 淋巴细胞与抗体
Humoral immunity involves B cells and antibodies dissolved in the blood and lymph. When a B cell binds to its specific antigen via its surface antibody, and receives stimulation from helper T cells, it becomes activated. Most activated B cells differentiate into plasma cells, which are antibody factories secreting thousands of antibodies per second. These antibodies bind to antigens, neutralising pathogens or marking them for destruction. A small number become memory B cells that provide long‑term immunity.
体液免疫涉及 B 细胞和溶解在血液和淋巴中的抗体。当 B 细胞通过其表面抗体与特定抗原结合，并接受辅助 T 细胞的刺激时，B 细胞被激活。大多数活化的 B 细胞分化为浆细胞，浆细胞是抗体工厂，每秒钟分泌数千个抗体。这些抗体与抗原结合，中和病原体或标记它们以供摧毁。少数成为记忆 B 细胞，提供长期免疫。
8. Antibody Structure and Function | 抗体的结构与功能
An antibody (immunoglobulin) is a Y‑shaped protein composed of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bonds. Each chain has a variable region at the tip of the arms, where the amino acid sequence varies greatly to form the antigen‑binding site, and a constant region that determines the antibody class. The variable region is specific to one antigen. Antibodies can agglutinate pathogens, neutralise toxins, precipitate soluble antigens, and act as opsonins to enhance phagocytosis.
抗体（免疫球蛋白）是一种 Y 形蛋白质，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。每条链在臂尖有一个可变区，该区域的氨基酸序列变化很大，形成抗原结合位点；还有一个恒定区，决定抗体的类别。可变区对一种抗原具有特异性。抗体能使病原体凝集、中和毒素、沉淀可溶性抗原，并作为调理素增强吞噬作用。
9. Active and Passive Immunity | 主动免疫与被动免疫
Active immunity is when the body produces its own antibodies and memory cells after exposure to an antigen, either through natural infection or vaccination. It is long‑lasting but takes time to develop. Passive immunity involves receiving antibodies from another source; this provides immediate protection but is temporary because no memory cells are produced. Examples include maternal antibodies crossing the placenta or antivenom injections.
主动免疫是指身体在接触抗原（通过自然感染或疫苗接种）后，自身产生抗体和记忆细胞。这种免疫力持久，但需要时间建立。被动免疫涉及从其他来源获得抗体；这能提供即时保护，但只是暂时的，因为没有产生记忆细胞。例子包括母体抗体穿过胎盘或抗蛇毒血清注射。
10. Vaccination and Herd Immunity | 疫苗接种与群体免疫
Vaccines contain antigens derived from pathogens, which may be live attenuated, inactivated, or subunit preparations. They stimulate a primary immune response without causing disease, producing memory cells. Upon later exposure to the actual pathogen, a rapid secondary response prevents illness. Herd immunity occurs when a high proportion of a population is vaccinated, reducing the spread of the pathogen and protecting those who cannot be vaccinated. Booster vaccinations are often needed to maintain antibody levels.
疫苗含有来自病原体的抗原，可以是减毒活疫苗、灭活疫苗或亚单位制剂。它们在不引起疾病的情况下刺激初次免疫反应，产生记忆细胞。当后来接触真正的病原体时，快速的二次反应能预防疾病。当人群中很高比例的人接种了疫苗，就会形成群体免疫，减少病原体传播并保护无法接种的人。通常需要加强注射以维持抗体水平。
11. Monoclonal Antibodies and Their Uses | 单克隆抗体及其用途
Monoclonal antibodies are identical antibodies produced from a single clone of plasma cells, usually hybridoma cells formed by fusing a plasma cell with a myeloma (cancer) cell. They are highly specific to one epitope on an antigen. In medicine, they are used in pregnancy tests (binding to hCG), in cancer therapy (targeting tumour markers), and in the treatment of autoimmune diseases. The ‘ELISA test’ uses monoclonal antibodies to detect either antigens or antibodies in a sample.
单克隆抗体是由单一克隆浆细胞（通常是由浆细胞与骨髓瘤细胞融合形成的杂交瘤细胞）产生的完全相同的抗体。它们对某个抗原上的单一表位具有高度特异性。在医学上，它们用于验孕（与 hCG 结合）、癌症治疗（靶向肿瘤标志物）以及自身免疫疾病的治疗。“ELISA 检测”利用单克隆抗体来检测样本中的抗原或抗体。
12. Autoimmune Diseases, Allergies and Immunodeficiency | 自身免疫疾病、过敏与免疫缺陷
When the immune system mistakenly attacks self‑cells, autoimmune diseases such as type 1 diabetes (destroying beta cells of the pancreas) or rheumatoid arthritis can develop. Allergies are hypersensitivity reactions to harmless environmental antigens (allergens), involving IgE antibodies and mast cells releasing histamine. Severe combined immunodeficiency (SCID) is a genetic disorder where both B and T cells are absent. The Human Immunodeficiency Virus (HIV) infects and destroys helper T cells, leading to Acquired Immune Deficiency Syndrome (AIDS), where the body becomes vulnerable to opportunistic infections.
当免疫系统错误地攻击自身细胞时，就会发生自身免疫疾病，如 1 型糖尿病（破坏胰腺 β 细胞）或类风湿性关节炎。过敏是对无害的环境抗原（过敏原）的超敏反应，涉及 IgE 抗体和肥大细胞释放组胺。严重联合免疫缺陷（SCID）是一种遗传疾病，患者缺失 B 细胞和 T 细胞。人类免疫缺陷病毒（HIV）感染并破坏辅助 T 细胞，导致获得性免疫缺陷综合征（AIDS），此时身体极易受到机会性感染的侵袭。
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