📚 IGCSE Biology: Immune System – Key Points Explained | IGCSE 生物：免疫系统考点精讲
The immune system is a complex network of cells, tissues and organs that defends the body against pathogens such as bacteria, viruses and fungi. In IGCSE Biology, you need to understand the difference between non-specific and specific defences, the role of lymphocytes in producing antibodies, and how vaccination can build immunity. This article breaks down the key concepts to help you revise efficiently.
免疫系统是一个由细胞、组织和器官组成的复杂网络，它保护身体免受细菌、病毒和真菌等病原体的侵害。在IGCSE生物学中，你需要理解非特异性防御和特异性防御的区别、淋巴细胞在产生抗体中的作用，以及疫苗接种如何建立免疫力。本文将分解关键概念，帮助你高效复习。
1. Overview of Body Defences | 身体防御概述
The human body has evolved a multi-layered defence system to prevent pathogens from causing disease. These layers are classified as non-specific (innate) defences and specific (adaptive) immunity. Non-specific defences include physical barriers, chemicals and phagocytic cells that act quickly regardless of the pathogen type. Specific immunity involves lymphocytes that target particular pathogens via antigen recognition, leading to the production of antibodies and immunological memory.
人体已进化出多层防御系统，防止病原体引发疾病。这些层次分为非特异性（先天）防御和特异性（适应性）免疫。非特异性防御包括物理屏障、化学物质和吞噬细胞，它们能迅速起作用，不区分病原体种类。特异性免疫则涉及淋巴细胞，通过识别抗原来靶向特定病原体，从而产生抗体和免疫记忆。
In IGCSE Biology, you may be asked to describe first, second and third lines of defence. Remember that the first two lines are non-specific, while the third line – the immune response involving lymphocytes – is specific and gives long-lasting protection.
在IGCSE生物学中，你可能会被要求描述第一、第二和第三道防线。请记住，前两道是非特异性的，而第三道——涉及淋巴细胞的免疫应答——是特异性的，并能提供持久的保护。
2. Non-Specific Defences: Barriers and Phagocytosis | 非特异性防御：屏障与吞噬作用
Physical barriers include the skin, which is a tough, waterproof layer of dead keratinised cells. The cilia and mucus in the respiratory tract trap pathogens and sweep them away. Chemical barriers include stomach acid (hydrochloric acid) that destroys swallowed pathogens, and lysozyme enzymes in tears and saliva that break down bacterial cell walls.
物理屏障包括皮肤，它是由死去的角质化细胞构成的坚韧防水层。呼吸道中的纤毛和黏液会捕捉病原体并将其扫走。化学屏障包括胃酸（盐酸），可消灭吞入的病原体；泪液和唾液中的溶菌酶能分解细菌细胞壁。
Phagocytosis: Phagocytes, such as neutrophils and macrophages, are white blood cells that carry out non-specific engulfment of pathogens. They detect, surround and ingest the pathogen into a phagosome, which then fuses with a lysosome containing digestive enzymes. The pathogen is broken down and the debris is released.
吞噬作用：吞噬细胞（如中性粒细胞和巨噬细胞）是进行非特异性吞噬病原体的白细胞。它们探测、包围并将病原体摄入形成吞噬体，吞噬体随后与含有消化酶的溶酶体融合。病原体被分解，残骸排出。
Inflammation is another non-specific response: damaged cells release histamine, causing blood vessels to dilate and become more permeable. This allows more phagocytes and fluid to reach the site of infection, resulting in redness, heat and swelling.
炎症是另一种非特异性反应：受损细胞释放组胺，导致血管扩张、通透性增加。这使得更多吞噬细胞和液体到达感染部位，引起红、热、肿。
3. Antigens and Specificity | 抗原与特异性
An antigen is any substance (usually a protein or polysaccharide on the surface of a pathogen) that is recognised by the immune system as foreign and triggers an immune response. Each antigen has a specific shape that is complementary to a particular antibody or lymphocyte receptor. This lock-and-key mechanism ensures that the immune system can target a vast diversity of pathogens with precision.
抗原是任何被免疫系统识别为异物并触发免疫应答的物质（通常是病原体表面的蛋白质或多糖）。每种抗原具有特定形状，与特定的抗体或淋巴细胞受体互补。这种锁钥机制确保了免疫系统能够精确靶向种类繁多的病原体。
The body’s own cells have self-antigens (MHC molecules) that prevent the immune system from attacking them. Inappropriate recognition of self-antigens can lead to autoimmune diseases.
身体自身的细胞带有自身抗原（MHC分子），可防止免疫系统攻击它们。对自身抗原的不当识别可能导致自身免疫性疾病。
4. B Lymphocytes and Antibody Production | B淋巴细胞与抗体产生
B lymphocytes (B cells) mature in the bone marrow. When a B cell encounters its specific antigen, it becomes activated, often with help from helper T cells. The activated B cell proliferates and differentiates into plasma cells and memory B cells. Plasma cells are antibody factories, secreting large amounts of antibodies into the blood and lymph.
B淋巴细胞（B细胞）在骨髓中成熟。当B细胞遇到其特定抗原时，会（通常借助辅助T细胞的帮助）被激活。激活的B细胞增殖并分化为浆细胞和记忆B细胞。浆细胞是抗体工厂，将大量抗体分泌到血液和淋巴中。
Antibodies (immunoglobulins) are Y-shaped proteins. They bind to antigens with high specificity, neutralising toxins, agglutinating (clumping) pathogens together, and marking them for phagocytosis (‘opsonisation’). Each B cell produces only one type of antibody.
抗体（免疫球蛋白）是Y形蛋白质。它们以高特异性结合抗原，可以中和毒素、使病原体凝集在一起，并标记它们以供吞噬（’调理作用’）。每个B细胞只产生一种类型的抗体。
The humoral immune response refers to antibody-mediated immunity, effective against pathogens present in body fluids (‘humors’).
体液免疫应答是指抗体介导的免疫，针对存在于体液（’humors’）中的病原体。
5. T Lymphocytes and Cell-Mediated Response | T淋巴细胞与细胞介导应答
T lymphocytes (T cells) mature in the thymus gland. They are responsible for cell-mediated immunity. There are two main types: helper T cells (CD4⁺), which activate B cells and cytotoxic T cells, and cytotoxic T cells (CD8⁺), which directly kill infected body cells.
T淋巴细胞（T细胞）在胸腺中成熟。它们负责细胞介导免疫。主要有两种类型：辅助T细胞（CD4⁺），可激活B细胞和细胞毒性T细胞；以及细胞毒性T细胞（CD8⁺），可直接杀死受感染的体细胞。
Antigen-presenting cells (such as macrophages) display pathogen antigens on their surface using MHC molecules. Helper T cells with matching receptors bind to this complex and secrete cytokines, stimulating the specific immune response. Cytotoxic T cells recognise infected cells and release perforin, which makes pores in the target cell membrane, leading to cell death.
抗原呈递细胞（如巨噬细胞）利用MHC分子在其表面展示病原体抗原。具有匹配受体的辅助T细胞与此复合物结合，并分泌细胞因子，刺激特异性免疫应答。细胞毒性T细胞识别受感染细胞，释放穿孔素，在靶细胞膜上打孔，导致细胞死亡。
Thus, T cells are crucial for eliminating intracellular pathogens such as viruses.
因此，T细胞对于清除病毒等细胞内病原体至关重要。
6. Primary vs Secondary Immune Response | 初次与二次免疫应答
When the body first encounters a pathogen, the primary immune response takes place. It involves the activation of lymphocytes, antibody production by plasma cells, and the generation of memory cells. The response is relatively slow (several days), and the antibody concentration peaks lower and then declines.
当身体初次遇到病原体时，会发生初次免疫应答。包括淋巴细胞的激活、浆细胞产生抗体以及产生记忆细胞。应答相对较慢（几天），抗体浓度达到峰值较低，然后下降。
The secondary immune response occurs upon re-exposure to the same antigen. Memory B and T cells recognise the antigen quickly and mount a faster, stronger and longer-lasting response. Antibody levels rise rapidly to a much higher concentration, often preventing symptoms. This is the basis of immunological memory and vaccination.
二次免疫应答发生在再次接触相同抗原时。记忆B细胞和记忆T细胞快速识别抗原，并启动更快、更强、更持久的应答。抗体水平迅速升至更高的浓度，通常能防止出现症状。这是免疫记忆和疫苗接种的基础。
| Feature | Primary Response | Secondary Response |
|---|---|---|
| Onset | Slow (several days) | Fast (hours to 1–2 days) |
| Antibody peak | Low and short-lived | High and sustained |
| Cells involved | 更多咨询请联系16621398022（同微信）
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