📚 Immune System Key Points for GCSE CCEA Biology | GCSE CCEA 生物：免疫系统 考点精讲
The human body has a complex defence network known as the immune system, which protects against pathogens such as bacteria, viruses, fungi and parasites. Understanding how our body recognises and destroys invaders is essential for GCSE CCEA Biology. This article reviews key concepts including barriers, phagocytosis, lymphocytes, antibodies, vaccination and types of immunity.
人体拥有一个复杂的防御网络，称为免疫系统，它能防御细菌、病毒、真菌和寄生虫等病原体。理解身体如何识别和消灭入侵者是 GCSE CCEA 生物学的重点。本文回顾关键概念，包括屏障、吞噬作用、淋巴细胞、抗体、疫苗接种和免疫类型。
1. Overview of Defence Systems | 防御系统概述
The immune system can be divided into non-specific (innate) defences and specific (adaptive) defences. Non-specific defences act rapidly against any pathogen, while specific defences target particular pathogens with precision.
免疫系统可分为非特异性（先天）防御和特异性（适应性）防御。非特异性防御能迅速对任何病原体作出反应，而特异性防御则精确针对特定的病原体。
Physical barriers like skin and mucous membranes form the first line of defence. If these are breached, internal defences such as phagocytes and lymphocytes are activated.
诸如皮肤和黏膜的物理屏障构成第一道防线。一旦它们被突破，吞噬细胞和淋巴细胞等内部防御机制就会被激活。
2. Physical and Chemical Barriers | 物理和化学屏障
The skin provides a physical barrier that is difficult for microbes to penetrate. It also produces sebum, an oily secretion that contains antimicrobial substances.
皮肤提供了一道微生物难以穿透的物理屏障。皮肤还分泌皮脂，这是一种含有抗微生物物质的油性分泌物。
The respiratory tract is lined with mucus-secreting ciliated epithelial cells. Mucus traps pathogens, and cilia sweep the mucus toward the throat to be swallowed – stomach acid then destroys the trapped microbes.
呼吸道内壁覆盖着分泌黏液并带有纤毛的上皮细胞。黏液能捕获病原体，纤毛将黏液推向喉部以便吞咽，随后胃酸会消灭被捕获的微生物。
Tears and saliva contain lysozyme, an enzyme that breaks down bacterial cell walls. The acidic environment of the stomach (HCl) also kills many ingested pathogens.
泪液和唾液中含有溶菌酶，这是一种能分解细菌细胞壁的酶。胃内的酸性环境（盐酸）也能杀死许多咽下的病原体。
3. Phagocytosis: A Non-Specific Cellular Response | 吞噬作用：非特异性细胞反应
Phagocytosis is a process by which certain white blood cells, called phagocytes (e.g. neutrophils and macrophages), engulf and digest pathogens.
吞噬作用是称为吞噬细胞（如中性粒细胞和巨噬细胞）的某些白细胞吞噬并消化病原体的过程。
The phagocyte is attracted by chemicals released by pathogens or damaged cells. It extends pseudopodia to surround the pathogen, enclosing it in a phagocytic vacuole. Digestive enzymes (lysozymes) then break down the microbe.
吞噬细胞被病原体或受损细胞释放的化学物质吸引。它伸出伪足包围病原体，将其困在吞噬小泡内。随后消化酶（溶菌酶）将微生物分解。
Phagocytosis is non-specific because phagocytes will engulf any foreign particle that is recognised as ‘non-self’. The remains of the pathogen are expelled or displayed on the cell surface to alert lymphocytes.
吞噬作用是非特异性的，因为吞噬细胞会吞噬任何被识别为“非己”的外来颗粒。病原体的残余物要么被排出，要么被呈递在细胞表面以警示淋巴细胞。
4. Antigens and Antibodies: The Basis of Specific Immunity | 抗原与抗体：特异性免疫的基础
An antigen is a molecule (often a protein or polysaccharide) on the surface of a pathogen that the immune system recognises as foreign. Each pathogen has its own unique antigens.
抗原是病原体表面被免疫系统识别为外来物质的分子（通常是蛋白质或多糖）。每种病原体都有自身独特的抗原。
An antibody is a Y-shaped protein produced by B lymphocytes that binds specifically to a complementary antigen. This binding marks the pathogen for destruction by other immune cells or neutralises the pathogen directly.
抗体是由 B 淋巴细胞产生的 Y 形蛋白质，能特异性地与互补抗原结合。这种结合能标记病原体以供其他免疫细胞消灭，或者直接中和病原体。
The antibody-antigen fit is highly specific, like a lock and key. A single B cell produces only one type of antibody, ensuring a tailored response.
抗体与抗原的契合高度特异，如同锁和钥匙。一个 B 细胞只产生一种类型的抗体，从而确保针对性反应。
5. Lymphocytes: B Cells, T Cells and the Immune Response | 淋巴细胞：B 细胞、T 细胞与免疫应答
Lymphocytes are a type of white blood cell responsible for specific immunity. There are two main types: B lymphocytes (B cells) and T lymphocytes (T cells).
淋巴细胞是一类负责特异性免疫的白细胞。主要有两种类型：B 淋巴细胞（B 细胞）和 T 淋巴细胞（T 细胞）。
B cells mature in the bone marrow and mediate humoral immunity. When a B cell encounters its complementary antigen, it is activated (often with help from T helper cells), then divides to form plasma cells that secrete large quantities of antibodies, and memory cells.
B 细胞在骨髓中成熟，介导体液免疫。当 B 细胞遇到其互补的抗原时，它被激活（通常需要辅助性 T 细胞的帮助），然后分裂形成浆细胞和记忆细胞，前者分泌大量抗体。
T cells mature in the thymus and are involved in cell-mediated immunity. Cytotoxic T cells directly kill infected body cells displaying antigen fragments. Helper T cells release chemical signals (cytokines) that stimulate B cells and other immune cells.
T 细胞在胸腺中成熟，参与细胞介导免疫。细胞毒性 T 细胞能直接杀死表面展示抗原片段的受感染人体细胞。辅助性 T 细胞则释放化学信号（细胞因子）来刺激 B 细胞和其他免疫细胞。
6. Primary and Secondary Immune Responses | 初次和二次免疫应答
When the body is exposed to a pathogen for the first time, there is a time lag before sufficient antibodies are produced. This is the primary immune response; the person may show symptoms of the disease during this delay.
当身体首次接触某种病原体时，需要经过一段滞后期才能产生足够数量的抗体。这就是初次免疫应答；
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