📚 A-Level OCR Biology: Immune System Key Revision Points | A-Level OCR 生物：免疫系统 考点精讲
The immune system is a vital topic in OCR A-Level Biology, encompassing both non-specific innate defences and the specific adaptive responses that involve lymphocytes and antibodies. Understanding the mechanisms of cell-mediated and humoral immunity, antibody structure, immunological memory, and applications such as vaccination and monoclonal antibodies is essential for exam success.
免疫系统是 OCR A-Level 生物的重要主题，涵盖非特异性先天防御以及涉及淋巴细胞和抗体的特异性适应性应答。理解细胞介导免疫和体液免疫的机制、抗体结构、免疫记忆以及疫苗接种和单克隆抗体等应用对于考试成功至关重要。
1. Non-Specific (Innate) Defences | 非特异性（先天）防御
The body's first line of defence consists of physical and chemical barriers that prevent the entry of pathogens. These include the skin, mucous membranes, stomach acid, and enzymes like lysozyme in tears.
机体的第一道防线由阻止病原体进入的物理和化学屏障组成。这些包括皮肤、粘膜、胃酸以及泪液中的溶菌酶等酶。
Skin provides a mechanical barrier; it is tough and impermeable, and its outer layer of dead keratinised cells is difficult for pathogens to penetrate. Sebum secreted by sebaceous glands contains antimicrobial substances.
皮肤提供机械屏障；它坚韧且不透水，其外层死亡角化细胞难以被病原体穿透。皮脂腺分泌的皮脂含有抗菌物质。
Mucous membranes line the respiratory, digestive, and reproductive tracts, trapping pathogens in sticky mucus which is then swept away by cilia (e.g., in the trachea) or expelled by coughing/sneezing.
粘膜内衬于呼吸道、消化道和生殖道，将病原体粘附在粘稠的粘液中，然后由纤毛（例如气管中）扫除或通过咳嗽/打喷嚏排出。
Chemical defences include the acidic pH of the stomach (HCl) that denatures proteins in pathogens, and lysozyme in tears and saliva that breaks down bacterial cell walls.
化学防御包括胃的酸性 pH 值（HCl）使病原体中的蛋白质变性，以及泪液和唾液中的溶菌酶可分解细菌细胞壁。
2. Phagocytosis and Inflammation | 吞噬作用和炎症反应
When pathogens breach the outer barriers, the second line of non-specific defence involves phagocytic white blood cells, such as neutrophils and macrophages, and the inflammatory response.
当病原体突破外部屏障时，第二道非特异性防线涉及吞噬性白细胞，如中性粒细胞和巨噬细胞，以及炎症反应。
During phagocytosis, a phagocyte recognises foreign antigens on a pathogen, engulfs it by extending pseudopodia, and encloses it in a phagosome. Lysosomes then fuse with the phagosome, releasing digestive enzymes (lysozyme, proteases) that break down the pathogen. The phagocyte may present the pathogen's antigens on its surface via MHC molecules, acting as an antigen-presenting cell (APC).
在吞噬作用中，吞噬细胞识别病原体上的外来抗原，通过伸出伪足将其吞噬，并将其包裹在吞噬体内。随后溶酶体与吞噬体融合，释放消化酶（溶菌酶、蛋白酶）分解病原体。吞噬细胞可能通过 MHC 分子将病原体的抗原呈递在其表面，充当抗原呈递细胞（APC）。
Inflammation is triggered by damaged tissues releasing histamine, which causes local vasodilation and increased capillary permeability. This leads to redness, heat, swelling, and pain, and helps more phagocytes and antimicrobial compounds reach the site of infection.
炎症由受损组织释放组胺引发，组胺导致局部血管舒张和毛细血管通透性增加。这导致发红、发热、肿胀和疼痛，并有助于更多吞噬细胞和抗微生物化合物到达感染部位。
3. The Specific Immune Response: An Overview | 特异性免疫应答概述
The specific (adaptive) immune system targets particular pathogens and provides immunological memory. It involves two main types of lymphocytes: B cells and T cells, which recognise antigens via specific receptors.
特异性（适应性）免疫系统针对特定病原体并提供免疫记忆。它涉及两种主要淋巴细胞：B 细胞和 T 细胞，它们通过特异性受体识别抗原。
Antigens are molecules (usually proteins or polysaccharides on pathogen surfaces) that stimulate an immune response. Each lymphocyte carries receptors complementary to a single antigen; when it encounters the non-self antigen that fits, it is activated – a process known as clonal selection.
抗原是激发免疫应答的分子（通常为病原体表面的蛋白质或多糖）。每个淋巴细胞携带与单一抗原互补的受体；当它遇到匹配的非自身抗原时便会被激活——这一过程称为克隆选择。
The specific response is divided into cell-mediated immunity, involving T lymphocytes that destroy infected host cells, and humoral immunity, involving B lymphocytes that produce antibodies to eliminate pathogens in body fluids.
特异性应答分为细胞介导免疫（涉及摧毁受感染宿主细胞的 T 淋巴细胞）和体液免疫（涉及产生抗体以清除体液中病原体的 B 淋巴细胞）。
4. Cell-Mediated Immunity: T Lymphocytes | 细胞介导免疫：T 淋巴细胞
T lymphocytes (T cells) mature in the thymus gland. Their receptors (TCRs) recognise antigen fragments bound to MHC proteins on the surface of other cells. There are two main subsets: helper T cells (CD4+) and cytotoxic T cells (CD8+).
T 淋巴细胞（T 细胞）在胸腺成熟。它们的受体（TCR）识别与其它细胞表面 MHC 蛋白结合的抗原片段。主要有两个亚群：辅助性 T 细胞（CD4+）和细胞毒性 T 细胞（CD8+）。
Helper T cells are activated when their TCRs bind to antigen-MHC class II complexes on antigen-presenting cells (APCs) such as dendritic cells or macrophages. Once activated, they secrete cytokines (e.g., interleukins) that stimulate the proliferation of cytotoxic T cells, B cells, and further phagocytic activity.
辅助性 T 细胞当其 TCR 与抗原呈递细胞（如树突
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