📚 Immune System Key Points | 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, fungi and parasites. In both IB and AQA biology syllabuses, understanding the lines of defence, the roles of B and T lymphocytes, antibody structure, and the principles of vaccination is essential. This article distils the key concepts you need to master, with clear bilingual explanations and exam-focused details.
免疫系统是一个由细胞、组织和器官组成的复杂网络，负责防御细菌、病毒、真菌和寄生虫等病原体。在IB和AQA生物课程中，理解防线层次、B淋巴细胞和T淋巴细胞的功能、抗体结构以及疫苗接种原理至关重要。本文浓缩了你必须掌握的核心概念，提供清晰的双语解释和紧扣考点的细节。
1. Lines of Defence | 防线概述
The human body has three main lines of defence against infection. The first line consists of physical and chemical barriers that prevent pathogen entry, such as the skin, mucous membranes, tears and stomach acid. The second line is the non-specific (innate) immune response, involving phagocytes, inflammation and fever. The third line is the specific (adaptive) immune response, which targets particular pathogens using lymphocytes and produces immunological memory.
人体抵御感染有三道主要防线。第一道防线由阻止病原体进入的物理和化学屏障组成，例如皮肤、黏膜、眼泪和胃酸。第二道防线是非特异性（先天性）免疫反应，涉及吞噬细胞、发炎和发热。第三道防线是特异性（适应性）免疫反应，利用淋巴细胞靶向特定病原体，并产生免疫记忆。
2. Non-specific Immune Responses | 非特异性免疫反应
When pathogens breach the first line of defence, non-specific mechanisms are activated. Phagocytes, including neutrophils and macrophages, engulf and digest foreign particles in a process called phagocytosis. Inflammation occurs as damaged tissues release histamine, causing vasodilation and increased capillary permeability, which allows more white blood cells to reach the site. Fever can inhibit pathogen growth and speed up immune reactions, while interferons help limit viral spread.
当病原体突破第一道防线时，非特异性机制被激活。吞噬细胞（包括中性粒细胞和巨噬细胞）通过吞噬作用吞食并消化异物。炎症发生时，受损组织释放组胺，引起血管扩张和毛细血管通透性增加，使更多白细胞到达感染部位。发热能抑制病原体生长并加速免疫反应，而干扰素则有助于限制病毒传播。
3. Specific Immunity: An Overview | 特异性免疫概述
Specific immunity involves the recognition of antigens, which are specific molecules (often proteins or polysaccharides) on the surface of pathogens or foreign cells. Lymphocytes have receptor proteins that bind to complementary antigens. B cells are responsible for humoral immunity (antibody production), while T cells are responsible for cell-mediated immunity. Both types originate from stem cells in the bone marrow, but T cells mature in the thymus.
特异性免疫包括对抗原的识别；抗原是病原体或外来细胞表面的特定分子（通常是蛋白质或多糖）。淋巴细胞表面具有能与互补抗原结合的受体蛋白。B细胞负责体液免疫（产生抗体），T细胞负责细胞介导的免疫。两类细胞均起源于骨髓中的干细胞，但T细胞在胸腺中成熟。
4. Antigens and Antibodies | 抗原与抗体
An antigen is any substance that can stimulate an immune response, typically a foreign macromolecule. Antibodies (immunoglobulins) are Y-shaped protein molecules secreted by plasma cells (activated B cells). Each antibody has a variable region that binds specifically to an epitope on the antigen. The constant region determines the antibody class (IgG, IgM, IgA, IgE, IgD). Antibodies can neutralise pathogens, agglutinate them, or mark them for destruction by phagocytes (opsonisation).
抗原是能够激发免疫反应的任何物质，通常是外来大分子。抗体（免疫球蛋白）是由浆细胞（活化的B细胞）分泌的Y形蛋白质分子。每个抗体都有一个可变区，能特异性地与抗原上的一个表位结合。恒定区决定了抗体的类别（IgG、IgM、IgA、IgE、IgD）。抗体可以中和病原体、使其凝集，或标记它们以便吞噬细胞摧毁（调理作用）。
Antigen-binding site: variable region → H₂N – L chain – S–S – H chain – COOH
抗原结合位点：可变区 → H₂N – 轻链 – 二硫键 – 重链 – COOH
5. Humoral Immunity (B Cells) | 体液免疫（B细胞）
When a B cell encounters its specific complementary antigen, it engulfs and processes the antigen, then presents fragments on its surface using MHC class II molecules. This allows a helper T cell with the matching receptor to bind and release cytokines, which activate the B cell. The B cell then divides by mitosis to form a clone of plasma cells and memory cells. Plasma cells secrete large numbers of antibodies into the blood and lymph, providing immediate defence.
当B细胞遇到其特异性互补抗原时，它会吞噬并处理该抗原，然后利用MHC II类分子将抗原片段呈递在细胞表面。这使得具有匹配受体的辅助T细胞能够结合并释放细胞因子，从而激活B细胞。B细胞随后通过有丝分裂形成浆细胞和记忆细胞的克隆。浆细胞向血液和淋巴液中分泌大量抗体，提供即时防御。
6. Cell-mediated Immunity (T Cells) | 细胞介导免疫（T细胞）
Cell-mediated immunity is primarily directed against cells that have been infected by viruses or have become cancerous. Cytotoxic T cells (CD8+) recognise antigens presented by MHC class I molecules on infected body cells. Once activated, they release perforin and granzymes that induce apoptosis in the target cell. Helper T cells (CD4+) do not kill directly but orchestrate the immune response by activating B cells and cytotoxic T cells.
细胞介导免疫主要针对被病毒感染的细胞或已变为癌细胞的自身细胞。细胞毒性T细胞（CD8+）能识别被感染体细胞表面MHC I类分子所呈递的抗原。激活后，它们释放穿孔素和颗粒酶，诱导靶细胞凋亡。辅助T细胞（CD4+）不直接杀伤，而是通过激活B细胞和细胞毒性T细胞来协调免疫反应。
7. Antigen Presentation and MHC | 抗原呈递与主要组织相容性复合体
Major Histocompatibility Complex (MHC) molecules are proteins found on the surface of cells that display peptide fragments for recognition by T cells. MHC class I is present on nearly all nucleated cells and presents endogenous antigens (e.g., viral proteins) to cytotoxic T cells. MHC class II is found mainly on professional antigen-presenting cells such as dendritic cells, macrophages and B cells, and presents exogenous antigens to helper T cells. This distinction is critical in both IB and AQA exams.
主要组织相容性复合体（MHC）是细胞表面的蛋白质，用来展示肽段供T细胞识别。MHC I类分子存在于几乎所有有核细胞上，并向细胞毒性T细胞呈递内源性抗原（如病毒蛋白）。MHC II类分子主要存在于专职抗原呈递细胞如树突状细胞、巨噬细胞和B细胞上，并向辅助T细胞呈递外源性抗原。这一区别在IB和AQA考试中都很关键。
8. Primary and Secondary Immune Responses | 初次与再次免疫应答
The primary immune response occurs when the body first encounters a pathogen. It is relatively slow (days) and produces a low concentration of antibodies, mainly IgM, as the specific B and T cell clones expand. Memory cells are formed during this response. Upon re-exposure, the secondary immune response is much faster and more intense, with a higher antibody concentration dominated by IgG. This rapid reaction often prevents symptoms, illustrating the basis of immunological memory.
初次免疫应答发生在机体首次接触病原体时。此过程相对较慢（数天），产生的抗体浓度较低，主要为IgM，同时特异性的B细胞和T细胞克隆开始扩增。在此阶段会形成记忆细胞。再次接触同一病原体时，二次免疫应答更快更强，抗体浓度更高且以IgG为主。这种快速反应通常可防止出现症状，体现了免疫记忆的基础。
9. Vaccination and Active/Passive Immunity | 疫苗接种与主动/被动免疫
Active immunity results from the production of antibodies and memory cells by the individual’s own immune system. It can be natural (through infection) or artificial (through vaccination). Vaccines contain weakened, inactivated pathogens or parts of them, which trigger a primary response and memory cell formation without causing disease. Passive immunity involves the introduction of ready-made antibodies from another source, such as across the placenta, through breast milk, or by injection of antiserum. It provides immediate but temporary protection, with no memory cells produced.
主动免疫是由个体自身免疫系统产生抗体和记忆细胞得来的，可以是自然的（通过感染）或人工的（通过疫苗接种）。疫苗含有减弱或灭活的病原体或其组成部分，能够激发初次应答并形成记忆细胞，而不引起疾病。被动免疫涉及从外部引入现成的抗体，例如通过胎盘、母乳或注射抗血清。它提供即时但暂时的保护，且不产生记忆细胞。
10. Monoclonal Antibodies | 单克隆抗体
Monoclonal antibodies are identical antibodies produced by a single clone of B cells, all specific to the same epitope. They are generated by fusing a plasma cell with a tumour cell to create a hybridoma, which can divide indefinitely while secreting the desired antibody. Applications include pregnancy testing, cancer diagnosis (e.g., PSA for prostate cancer), and targeted drug delivery. Ethical and safety issues are also part of the syllabus.
单克隆抗体是由单一B细胞克隆产生的完全相同的抗体，均针对同一个抗原表位。它们通过将浆细胞与肿瘤细胞融合制成杂交瘤来生产，杂交瘤能无限分裂并持续分泌所需的抗体。应用领域包括妊娠检测、癌症诊断（如前列腺癌的PSA检测）以及靶向药物递送。伦理和安全问题也是课程大纲的一部分。
11. HIV and AIDS | HIV与艾滋病
Human Immunodeficiency Virus (HIV) is a retrovirus that infects helper T cells, macrophages and dendritic cells by binding to CD4 receptors. Over time, the loss of helper T cells severely weakens the immune system, making the individual susceptible to opportunistic infections and eventually leading to Acquired Immune Deficiency Syndrome (AIDS). Understanding the viral life cycle, including reverse transcriptase and integration, is emphasised, as are social and ethical considerations.
人类免疫缺陷病毒（HIV）是一种逆转录病毒，通过与CD4受体结合而感染辅助T细胞、巨噬细胞和树突状细胞。随着时间推移，辅助T细胞的丧失会严重削弱免疫系统，使个体容易受到机会性感染，最终导致获得性免疫缺陷综合征（艾滋病）。病毒生命周期（包括逆转录酶和整合）是重点，社会与伦理因素同样需要关注。
12. Autoimmune Diseases and Allergies | 自身免疫疾病与过敏
Sometimes the immune system malfunctions. Autoimmune diseases occur when the body fails to recognise self-antigens and launches an immune attack against its own tissues, as seen in Type 1 diabetes (destruction of pancreatic beta cells) or multiple sclerosis. Allergies are exaggerated immune responses to harmless environmental antigens (allergens). The allergen triggers plasma cells to produce IgE, which binds to mast cells; subsequent exposure leads to degranulation and histamine release, causing symptoms like inflammation and bronchoconstriction.
有时免疫系统会功能异常。自身免疫疾病发生在机体未能识别自身抗原并对自身组织发起免疫攻击时，如1型糖尿病（破坏胰腺β细胞）或多发性硬化症。过敏是对无害环境抗原（过敏原）的异常强烈的免疫反应。过敏原激发浆细胞产生IgE，IgE与肥大细胞结合；再次接触过敏原时导致肥大细胞脱颗粒并释放组胺，引起炎症和支气管收缩等症状。
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