📚 Immunity | 免疫
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. A-level Biology requires understanding both non-specific defences, which act rapidly against any foreign agent, and specific immune responses, which are slower but highly targeted and produce immunological memory.
免疫系统是一个复杂的细胞、组织和分子网络，负责保护机体免受细菌、病毒、真菌和寄生虫等病原体的侵害。A-level生物课程要求理解两类防御：非特异性防御能快速抵御任何外来物；特异性免疫应答较慢，但具有高度靶向性并能形成免疫记忆。
1. Overview of the Immune System | 免疫系统概述
The immune system can be divided into three lines of defence. The first line consists of physical and chemical barriers such as the skin, mucous membranes and stomach acid. The second line includes phagocytes and the inflammatory response, while the third line is the specific immune response carried out by lymphocytes.
免疫系统可以分为三道防线。第一道防线包括皮肤、黏膜和胃酸等物理和化学屏障。第二道防线包括吞噬细胞和炎症反应。第三道防线是由淋巴细胞执行的特异性免疫应答。
2. Non-Specific Defences | 非特异性防御
Non-specific defences do not distinguish between different pathogens. The skin provides a tough physical barrier and secretes sebum, which has antimicrobial properties. Mucous membranes in the respiratory tract trap microorganisms, and cilia sweep mucus upwards to the throat where it is swallowed. Stomach acid (hydrochloric acid) destroys many ingested pathogens, and lysozyme in tears and saliva breaks down bacterial cell walls.
非特异性防御不能区分不同病原体。皮肤提供坚固的物理屏障，并分泌具有抗菌作用的皮脂。呼吸道黏膜能捕获微生物，纤毛将黏液向上推送至咽部并吞入消化道。胃酸（盐酸）能杀死许多进入消化道的病原体；眼泪和唾液中的溶菌酶可分解细菌细胞壁。
3. Phagocytosis | 吞噬作用
Phagocytes, mainly neutrophils and macrophages, carry out phagocytosis. First, the phagocyte is attracted to the pathogen by chemicals and recognises it as foreign. The cell membrane engulfs the pathogen, forming a phagosome. Lysosomes fuse with the phagosome, releasing digestive enzymes and reactive oxygen species, which destroy the pathogen. The harmless breakdown products are absorbed or expelled by exocytosis.
吞噬细胞主要是中性粒细胞和巨噬细胞，执行吞噬作用。首先，吞噬细胞被病原体释放的化学物质吸引并识别其为外来物。细胞膜包裹病原体形成吞噬体。溶酶体与吞噬体融合，释放消化酶和活性氧物质，将病原体破坏。无害的分解产物被吸收或通过胞吐作用排出。
4. Antigens and Antibodies | 抗原与抗体
Antigens are molecules, usually proteins or polysaccharides on the surface of pathogens, that the immune system recognises as foreign. Antibodies are Y-shaped glycoproteins called immunoglobulins, produced by plasma cells. Each antibody has a variable region that binds specifically to one antigen, and a constant region that determines its class and effector function. The variable region gives antibodies their specificity.
抗原通常是病原体表面的蛋白质或多糖分子，免疫系统将其识别为外来物质。抗体是由浆细胞产生的Y形糖蛋白，称为免疫球蛋白。每个抗体具有可变区，能够特异性结合某一抗原；恒定区决定抗体的类别和效应功能。可变区赋予抗体特异性。
The main functions of antibodies include neutralisation of toxins and viruses, agglutination of bacteria to make them easier to phagocytose, opsonisation (coating pathogens to enhance phagocytosis) and activation of the complement system.
抗体的主要功能包括：中和毒素和病毒、凝集细菌使其更易被吞噬、调理作用（包被病原体以增强吞噬）以及激活补体系统。
5. B Lymphocytes and Humoral Immunity | B淋巴细胞与体液免疫
Humoral immunity involves B lymphocytes and the production of antibodies in the blood and lymph. A specific B cell has receptors that bind to a complementary antigen. With help from T helper cells, the B cell becomes activated, undergoes clonal selection and divides rapidly. Some daughter cells differentiate into short-lived plasma cells, which secrete large amounts of antibodies. Others become long-lived memory cells that remain in the body for rapid response to future infections.
体液免疫涉及B淋巴细胞以及在血液和淋巴中产生抗体。特定的B细胞表面受体能够结合互补的抗原。在辅助性T细胞的帮助下，B细胞被激活，发生克隆选择并快速分裂。部分子细胞分化为寿命较短的浆细胞，分泌大量抗体；另一些成为长寿的记忆细胞，存留在体内以便将来感染时快速应答。
6. T Lymphocytes and Cell-Mediated Immunity | T淋巴细胞与细胞介导免疫
Cell-mediated immunity is carried out by T lymphocytes. T helper cells (CD4+) recognise antigen fragments presented on the surface of antigen-presenting cells by MHC class II molecules. They release cytokines that activate B cells, cytotoxic T cells and phagocytes. Cytotoxic T cells (CD8+) recognise infected body cells presenting viral antigens on MHC class I molecules and kill them by releasing perforin and granzymes, which induce apoptosis.
细胞介导免疫由T淋巴细胞执行。辅助性T细胞（CD4+）识别抗原呈递细胞表面由MHC II类分子呈递的抗原片段。它们释放细胞因子，激活B细胞、细胞毒性T细胞和吞噬细胞。细胞毒性T细胞（CD8+）识别被感染细胞表面由MHC I类分子呈递的病毒抗原，通过释放穿孔素和颗粒酶诱导细胞凋亡将其杀死。
7. Primary and Secondary Immune Responses | 初次与二次免疫应答
After first exposure to an antigen, the primary immune response is slow, taking several days to produce antibodies, and the antibody concentration peaks at a relatively low level. Memory cells are produced during this response. On second exposure, the secondary immune response is faster and stronger, with a much higher peak antibody concentration, often preventing symptoms before they appear. This is due to clonal expansion of memory B and T cells.
初次接触抗原后，初次免疫应答较慢，需要数天才能产生抗体，抗体浓度峰值相对较低。此过程中产生记忆细胞。再次接触相同抗原时，二次免疫应答更快更强，抗体浓度峰值更高，常在症状出现前就清除病原体。这是因为记忆B细胞和T细胞能够快速克隆扩增。
8. Types of Immunity: Active and Passive | 免疫类型：主动免疫与被动免疫
Active immunity results from the body producing its own antibodies and memory cells, either after natural infection or after vaccination. Passive immunity involves receiving antibodies from another source, such as maternal antibodies crossing the placenta or through breast milk, or by injection of antibody serum. Active immunity is long-lasting, whereas passive immunity provides immediate but short-term protection because the antibodies are eventually broken down and no memory cells are produced.
主动免疫由机体自身产生抗体和记忆细胞获得，可以通过自然感染或疫苗接种产生。被动免疫是从外部获得抗体，例如母体抗体穿过胎盘或通过母乳传递，或注射抗体血清。主动免疫持续时间长；被动免疫能提供即时但短期的保护，因为外来抗体会被降解且不产生记忆细胞。
9. Vaccination and Herd Immunity | 疫苗接种与群体免疫
A vaccine contains antigens from a pathogen, often in the form of dead or weakened microorganisms, toxoids, or subunit proteins. Vaccination triggers a primary immune response and the production of memory cells without causing disease. Herd immunity occurs when a sufficiently high percentage of a population is immunised, reducing the spread of the pathogen and protecting vulnerable individuals who cannot be vaccinated, such as newborns or immunocompromised patients.
疫苗含有病原体的抗原，通常采用灭活或减毒微生物、类毒素或亚单位蛋白等形式。疫苗接种可诱导初次免疫应答并产生记忆细胞，但不会引起疾病。当人群中有足够高比例的人接种疫苗时，就会形成群体免疫，从而减少病原体传播，保护无法接种疫苗的易感人群，如新生儿或免疫功能低下者。
Some vaccines require booster doses because antibody levels decline over time or because the pathogen mutates, as seen with influenza.
有些疫苗需要加强针，因为抗体水平会随时间下降，或病原体会发生变异，例如流感病毒。
10. Autoimmune Diseases and Allergies | 自身免疫病与过敏
Autoimmune diseases arise when the immune system fails to distinguish self from non-self and attacks the body’s own cells. Examples include type 1 diabetes, in which insulin-producing beta cells of the pancreas are destroyed, and multiple sclerosis, in which the myelin sheath of neurones is damaged. Allergies are hypersensitive responses to harmless environmental antigens called allergens, such as pollen or dust mite faeces. The first exposure leads to production of IgE antibodies, which bind to mast cells. On re-exposure, allergens cross-link the IgE, triggering mast cells to release histamine, causing inflammation and symptoms such as sneezing, itching and bronchoconstriction.
当免疫系统无法区分自身和非自身并攻击机体自身细胞时，就会发生自身免疫病。例如1型糖尿病中，产生胰岛素的胰岛β细胞被破坏；多发性硬化症中，神经元的髓鞘受损。过敏是对无害环境抗原（如花粉或尘螨粪便）的过度敏感反应。首次接触过敏原会诱导产生IgE抗体，IgE结合到肥大细胞表面。再次接触时，过敏原使IgE交联，促使肥大细胞释放组胺，引起炎症和打喷嚏、瘙痒、支气管收缩等症状。
11. Monoclonal Antibodies | 单克隆抗体
Monoclonal antibodies are identical antibodies produced by hybridoma cells. A B lymphocyte producing the desired antibody is fused with a myeloma (cancer) cell to form a hybridoma, which divides indefinitely and secretes large quantities of a single antibody. They are used in medical diagnosis, such as in pregnancy tests to detect human chorionic gonadotropin (hCG), and in treatment, for example, targeting cancer cells while sparing healthy tissue.
单克隆抗体是由杂交瘤细胞产生的完全相同抗体。将产生所需抗体的B淋巴细胞与骨髓瘤（癌细胞）融合，形成杂交瘤细胞，能无限分裂并分泌大量单一抗体。它们用于医学诊断，例如早孕试纸检测人绒毛膜促性腺激素（hCG）；也用于治疗，例如靶向癌细胞而不损伤健康组织。
12. Exam Tips for Immunity | 备考技巧
In A-level Biology exams, make sure you can distinguish between phagocytes and lymphocytes, between B cells and T cells, and between active and passive immunity. Use precise terms such as ‘clonal selection’, ‘plasma cell’, ‘memory cell’, ‘antigen presentation’, ‘MHC’ and ‘cytokines’. Common errors include stating that antibodies kill pathogens directly (they do not; they mark or neutralise them), or claiming that vaccines contain antibodies (they contain antigens).
在A-level生物考试中，要能够区分吞噬细胞与淋巴细胞、B细胞与T细胞、主动免疫与被动免疫。使用精确术语，如“克隆选择”、“浆细胞”、“记忆细胞”、“抗原呈递”、“MHC”和“细胞因子”。常见错误包括声称抗体直接杀死病原体（实际上它们只是标记或中和病原体），或认为疫苗含有抗体（实际上疫苗含有抗原）。
When describing phagocytosis, use the terms chemotaxis, endocytosis, phagosome, lysosome fusion and exocytosis. When comparing primary and secondary responses, refer to the speed, magnitude and duration of antibody concentration.
描述吞噬作用时，应使用趋化作用、内吞、吞噬体、溶酶体融合和胞吐等术语。比较初次和二次应答时，要提到抗体浓度变化的速度、幅度和持续时间。
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