Human Defence Mechanisms Against Disease | 人体防御疾病的免疫机制

📚 Human Defence Mechanisms Against Disease | 人体防御疾病的免疫机制

The human body is constantly exposed to pathogens such as bacteria, viruses, fungi and parasites. To survive in this hostile environment, we rely on a highly organised immune system that detects and eliminates foreign invaders while leaving our own cells unharmed.

人体时刻暴露于细菌、病毒、真菌和寄生虫等病原体之中。为了在这样恶劣的环境中生存，我们依赖一套高度有序的免疫系统来识别并清除外来入侵者，同时不伤害自身细胞。

This immune system operates at several levels, from simple barriers to sophisticated memory-based responses. In this article, we will examine the mechanisms of human defence against disease, following the CIE A-Level Biology specification.

这一免疫系统在多个层面发挥作用，从简单的屏障到基于记忆的复杂应答。本文将依据 CIE A-Level 生物考纲，探讨人体防御疾病的机制。

1. Physical and Chemical Barriers | 第一道防线：物理和化学屏障

The first line of defence is non-specific and includes all the barriers that physically or chemically prevent pathogens from entering the tissues. The skin is the largest organ and provides a tough, dry and waterproof outer surface. Its outer layer, the stratum corneum, is made of dead keratinised cells that are constantly shed, taking attached microorganisms with them.

第一道防线是非特异性的，包括所有从物理或化学层面阻止病原体进入组织的屏障。皮肤是最大的器官，提供坚韧、干燥和防水的体表。其外层角质层由不断脱落的死角质化细胞构成，并将附着的微生物一并带走。

Mucous membranes line the respiratory, digestive and reproductive tracts. They secrete sticky mucus that traps pathogens, while ciliated cells in the airways waft the mucus up to the throat for swallowing and destruction by stomach acid. In addition, stomach acid, lysozyme in tears and sweat, and antimicrobial peptides in skin secretions act as chemical barriers.

呼吸道、消化道和生殖道内衬有黏膜。黏膜分泌粘稠的粘液以粘住病原体，而气道中的纤毛细胞将粘液向上推送至咽部，使其被吞入并由胃酸杀灭。此外，胃酸、眼泪和汗液中的溶菌酶以及皮肤分泌物中的抗菌肽也起到化学屏障作用。

Physical barriers include skin, mucous membranes, cilia, and the flushing action of urine and tears. 物理屏障包括皮肤、黏膜、纤毛以及尿液和泪液的冲刷作用。
Chemical barriers include acidic pH, sebum, lysozyme, and antimicrobial proteins. 化学屏障包括酸性环境、皮脂、溶菌酶和抗菌蛋白。

These barriers are non-specific; they act against all pathogens equally and do not distinguish between different types of microorganism.

这些屏障均为非特异性，它们对所有病原体同等作用，并不区分不同种类的微生物。

2. Phagocytosis and the Second Line of Defence | 吞噬作用与第二道防线

When pathogens break through the first line of defence, the body activates the second line of defence, which is also innate and non-specific. Phagocytosis is a key mechanism in which white blood cells such as neutrophils and macrophages engulf and destroy pathogens.

当病原体突破第一道防线后，机体便激活同样属于先天且非特异性的第二道防线。吞噬作用是其中关键机制，由中性粒细胞和巨噬细胞等白细胞吞噬并摧毁病原体。

During phagocytosis, the phagocyte is attracted to the pathogen by chemicals such as complement proteins and cytokines. It then binds to the pathogen, extends pseudopodia around it, and internalises it in a phagosome. The phagosome fuses with a lysosome to form a phagolysosome, where lytic enzymes and reactive oxygen species kill and digest the invader.

在吞噬过程中，吞噬细胞受补体蛋白和细胞因子等化学信号吸引而趋向病原体。随后与病原体结合并伸出伪足将其包裹，以内吞形式纳入吞噬体。吞噬体与溶酶体融合形成吞噬溶酶体，其中的裂解酶和活性氧杀死并消化入侵者。

The second line of defence also includes inflammation. Damaged cells release histamine, which causes vasodilation and increased capillary permeability. This leads to redness, heat, swelling and pain, and allows more immune cells to reach the infected area. Fever, induced by pyrogens, can inhibit pathogen growth and improve immune efficiency.

第二道防线还包括炎症反应。受损细胞释放组胺，引起血管扩张和毛细血管通透性增加，从而出现红、热、肿、痛等表现，并允许更多免疫细胞到达感染部位。由致热原引发的发热可抑制病原体增殖并提高免疫效率。

Interferons are proteins released by virus-infected cells; they protect neighbouring cells by inhibiting viral replication. Complement proteins can directly lyse pathogens and enhance phagocytosis by opsonisation.

干扰素是受病毒感染的细胞释放的蛋白质，可通过抑制病毒复制来保护邻近细胞。补体蛋白能直接裂解病原体，并通过调理作用增强吞噬。

3. The Adaptive Immune System: Antigens and Specificity | 适应性免疫系统：抗原与特异性

The third line of defence is the adaptive immune system, which is highly specific and has memory. It recognises and attacks particular pathogens after an initial encounter, and responds more rapidly on subsequent exposure.

第三道防线是适应性免疫系统，具有高度特异性和记忆性。它在初次接触后识别并攻击特定病原体，并在再次暴露时作出更快响应。

An antigen is any molecule, usually a protein or polysaccharide, that is recognised by the immune system as foreign. Most antigens are large and have many antigenic determinants, called epitopes, that bind to specific receptors on lymphocytes. Antigens can be found on the surface of pathogens, on infected cells, or on transplanted tissues.

抗原通常是蛋白质或多糖分子，可被免疫系统识别为外来物质。大多数抗原较大，带有多个被称为表位的抗原决定簇，可与淋巴细胞表面的特异性受体结合。抗原可存在于病原体表面、受感染细胞表面或移植组织上。

The adaptive immune system has two main branches: humoral immunity, mediated by B lymphocytes and antibodies, and cell-mediated immunity, mediated by T lymphocytes. Both arise from a common lymphoid stem cell in the bone marrow; B cells mature in the bone marrow, while T cells mature in the thymus.

适应性免疫系统分为两大分支：由 B 淋巴细胞和抗体介导的体液免疫，以及由 T 淋巴细胞介导的细胞免疫。两者都起源于骨髓中的共同淋巴干细胞；B 细胞在骨髓中成熟，T 细胞在胸腺中成熟。

Humoral immunity targets pathogens outside cells, such as bacteria in the bloodstream. 体液免疫针对细胞外的病原体，例如血液中的细菌。
Cell-mediated immunity targets infected cells or abnormal cells, such as virus-infected or cancer cells. 细胞免疫针对受感染细胞或异常细胞，如病毒感染的细胞或癌细胞。

4. B Lymphocytes and Humoral Immunity | B 淋巴细胞与体液免疫

B lymphocytes are responsible for producing antibodies, which are globular glycoproteins known as immunoglobulins. Each B cell has a unique B-cell receptor that recognises a specific antigen epitope. When an antigen binds to the BCR, the B cell becomes activated, often with the help of helper T cells.

B 淋巴细胞负责产生抗体，抗体是一类被称为免疫球蛋白的球状糖蛋白。每个 B 细胞具有独特的 B 细胞受体，可识别特定的抗原表位。当抗原与 BCR 结合时，该 B 细胞被激活，此过程通常需要辅助 T 细胞的协助。

Upon activation, the B cell divides repeatedly by mitosis, producing a clone of genetically identical cells. Some differentiate into plasma cells, which secrete large quantities of antibody into the bloodstream. Others become memory B cells, which remain in the body for years or even a lifetime, providing long-term immunity.

激活后，B 细胞通过有丝分裂反复增殖，产生遗传上相同的细胞克隆。其中一部分分化为浆细胞，向血液中分泌大量抗体；另一部分成为记忆 B 细胞，在体内存活数年甚至终身，提供长期免疫力。

Antibodies neutralise pathogens in several ways. They may bind directly to antigens on the surface of viruses or bacteria, preventing them from entering host cells. They can also agglutinate pathogens by cross-linking them into clumps, making them easier targets for phagocytes. In addition, antibodies act as opsonins, enhancing phagocytosis, and they can activate the complement system to cause cell lysis.

抗体通过多种方式中和病原体。它们可以直接结合病毒或细菌表面的抗原，阻止其进入宿主细胞；也可以通过交联形成凝集块，使其更易被吞噬细胞吞噬。此外，抗体还可充当调理素增强吞噬作用，并激活补体系统导致细胞裂解。

Antibody structure: Y-shaped, variable region for antigen binding, constant region for effector functions

抗体结构：Y 形，可变区负责结合抗原，恒定区负责效应功能

5. T Lymphocytes and Cell-Mediated Immunity | T 淋巴细胞与细胞免疫

T lymphocytes do not produce antibodies. Instead, they act directly on infected or abnormal cells. There are two major types: helper T cells (CD4⁺) and cytotoxic T cells (CD8⁺).

T 淋巴细胞不产生抗体，而是直接作用于受感染或异常细胞。主要有两种类型：辅助 T 细胞（CD4⁺）和细胞毒性 T 细胞（CD8⁺）。

Helper T cells recognise antigens presented on the surface of antigen-presenting cells in association with MHC class II molecules. Once activated, they secrete cytokines such as interleukins, which stimulate B cells to divide and produce antibodies, activate cytotoxic T cells, and enhance the activity of phagocytes. Because of this central role, helper T cells are sometimes called orchestrators of the immune response.

辅助 T 细胞识别抗原呈递细胞表面与 MHC II 类分子结合的抗原。激活后，它们分泌白介素等细胞因子，刺激 B 细胞分裂并产生抗体，激活细胞毒性 T 细胞，并增强吞噬细胞活性。由于这一核心作用，辅助 T 细胞有时被称为免疫应答的“指挥者”。

Cytotoxic T cells recognise antigens presented on MHC class I molecules, which are found on almost all nucleated cells. When an infected cell displays pathogen-derived peptides on its surface, a cytotoxic T cell binds to the peptide-MHC complex and releases perforin and granzymes. Perforin creates pores in the target cell membrane, while granzymes enter the cell and trigger apoptosis, or programmed cell death.

细胞毒性 T 细胞识别大多数有核细胞表面 MHC I 类分子所呈递的抗原。当受感染细胞在表面展示病原体来源的肽段时，细胞毒性 T 细胞便与该肽-MHC 复合物结合，并释放穿孔素和颗粒酶。穿孔素在靶细胞膜上形成孔道，颗粒酶进入细胞并触发凋亡，即程序性细胞死亡。

Cell-mediated immunity is essential for eliminating virally infected cells, intracellular bacteria, fungi and transplanted tissues. It also plays a role in killing cancer cells.

细胞免疫对于清除病毒感染细胞、胞内细菌、真菌以及移植组织至关重要，同时也参与杀死癌细胞。

6. Cooperation Between B and T Cells | B 细胞与 T 细胞的协作

Effective immune responses require coordinated action between different white blood cells. Antigen-presenting cells, such as dendritic cells and macrophages, ingest pathogens and present their peptide fragments on MHC class II molecules to helper T cells.

有效的免疫应答需要不同类型白细胞之间的协调行动。树突状细胞和巨噬细胞等抗原呈递细胞吞噬病原体，并将其肽段呈递在 MHC II 类分子上，供辅助 T 细胞识别。

This interaction is controlled by clonal selection. When a pathogen enters the body, only the few lymphocytes with receptors that fit the antigen are selected; they undergo clonal expansion, producing large numbers of effector cells. This process explains why the immune response takes time to develop during the primary infection.

这种相互作用受到克隆选择的调控。当病原体进入体内时，只有少数受体能与抗原匹配的淋巴细胞被选择出来；它们进行克隆扩增，产生大量效应细胞。这一过程解释了初次感染时免疫应答为何需要较长时间才能建立。

Helper T cells activate B cells in a T-dependent manner. The B cell presents processed antigen on MHC class II to the helper T cell, which then delivers cytokines to stimulate B cell proliferation and differentiation. Some antigens can activate B cells without T cell help, but T-dependent responses generally produce stronger antibodies and memory.

辅助 T 细胞以依赖 T 细胞的方式激活 B 细胞。B 细胞将加工后的抗原呈递在 MHC II 类分子上供辅助 T 细胞识别，随后辅助 T 细胞释放细胞因子，刺激 B 细胞增殖和分化。有些抗原可不依赖 T 细胞直接激活 B 细胞，但依赖 T 细胞的应答通常产生更强的抗体和记忆。

Memory cells generated during the initial response are crucial for long-term immunity. On subsequent exposure to the same antigen, memory cells are rapidly reactivated and differentiate into effector cells, producing a larger and faster secondary response.

初次应答中产生的记忆细胞是长期免疫的关键。当再次接触同一抗原时，记忆细胞被快速激活并分化为效应细胞，产生更强大更迅速的二次应答。

7. Primary and Secondary Immune Responses | 初次免疫应答与二次免疫应答

The first exposure to an antigen produces a primary immune response. There is a lag phase of several days while B and T cells undergo clonal selection and expansion. Then antibody levels slowly rise, reach a peak, and gradually decline as the pathogen is cleared.

首次接触抗原会产生初次免疫应答。在 B 细胞和 T 细胞进行克隆选择和扩增期间，存在数天的延迟期。随后抗体水平缓慢上升，达到峰值，并随病原体被清除而逐渐下降。

During the primary response, memory cells are formed. The second exposure to the same antigen triggers a secondary immune response, which is much more rapid and intense. Because memory cells are abundant and already specific, antibody production begins almost immediately and reaches a much higher level.

初次应答期间会形成记忆细胞。第二次接触同一抗原会触发二次免疫应答，速度更快、强度更大。由于记忆细胞数量多且已具特异性，抗体产生几乎立即开始，并达到更高水平。

This difference explains why many diseases cause illness only the first time and why vaccination can protect against future infections. It is also the basis for booster vaccinations, which deliberately stimulate the secondary response.

这种差异解释了为何许多疾病只在初次感染时引起症状，以及为何疫苗接种可以预防未来感染。这也是加强免疫的基础，即有意激发二次应答。

8. Active and Passive Immunity | 主动免疫与被动免疫

Immunity can be acquired either actively or passively, and each can be achieved naturally or artificially. Active immunity results from exposure to an antigen, leading the body to produce its own antibodies and memory cells. Passive immunity occurs when ready-made antibodies are transferred from another source, providing immediate but temporary protection.

免疫可以通过主动或被动方式获得，且每种方式都有天然和人工之分。主动免疫是接触抗原后，机体自身产生抗体和记忆细胞；被动免疫则是将从其他来源获得的现成抗体转移给机体，提供即时但短暂的保护。

Natural active immunity occurs when a person is naturally infected and recovers, leaving behind memory cells. Artificial active immunity is vaccination, where a weakened or inactivated pathogen is introduced to trigger an immune response without causing significant disease.

天然主动免疫发生在人自然感染并康复后，体内留下记忆细胞。人工主动免疫即疫苗接种，通过引入减毒或灭活的病原体，在不引起严重疾病的情况下激发免疫应答。

Natural passive immunity involves the transfer of antibodies from a mother to her fetus across the placenta, or to an infant through breast milk. Artificial passive immunity is the injection of antibodies, such as antiserum, to provide immediate protection after exposure to a dangerous pathogen, such as tetanus or rabies.

天然被动免疫包括母体抗体通过胎盘传递给胎儿，或通过母乳传递给婴儿。人工被动免疫是注射抗体，如抗血清，在接触破伤风或狂犬病等危险病原体后立即提供保护。

Type	Origin	Duration
Active immunity	Own immune system after antigen exposure	Long-term, memory cells
Passive immunity	Antibodies from another individual	Short-term, no memory

Table: Comparison of active and passive immunity

表：主动免疫与被动免疫的比较

9. Vaccines and Herd Immunity | 疫苗与群体免疫

Vaccination is the deliberate stimulation of a person’s immune system to produce protective immunity against a specific pathogen. A vaccine contains an antigen that is either attenuated, inactivated, or a part of the pathogen, such as a subunit protein or toxoid.

疫苗接种是有意刺激人体免疫系统，使其对特定病原体产生保护性免疫。疫苗含有减毒、灭活或部分病原体成分，如亚单位蛋白或类毒素。

Live attenuated vaccines use weakened pathogens that can replicate slowly but do not cause severe disease. They produce strong and lasting immunity. Inactivated vaccines contain killed pathogens and are safer but often require booster doses. Subunit vaccines, toxoid vaccines, and newer mRNA-based vaccines (as used for COVID-19) provide the antigen without intact pathogens.

减毒活疫苗使用弱化病原体，可在体内缓慢复制但不引起严重疾病，能产生较强而持久的免疫。灭活疫苗含有被杀死的病原体，安全性高但通常需要加强剂量。亚单位疫苗、类毒素疫苗以及新兴的 mRNA 疫苗（如 COVID-19 所用）提供抗原而不含完整病原体。

Herd immunity occurs when a sufficiently large proportion of the population is immune, so that the spread of pathogen is reduced and protected vulnerable individuals are less likely to be exposed. The threshold depends on the basic reproduction number (R₀) of the pathogen and the vaccine efficacy.

群体免疫是指当人群中足够大比例具备免疫力时，病原体传播受到限制，使易感或脆弱个体较少暴露于病原体。该阈值取决于病原体的基本传染数（R₀）和疫苗效力。

Vaccination programmes aim to reduce incidence and prevalence of infectious disease. 疫苗接种计划旨在降低感染性疾病的发病率和患病率。
Herd immunity protects those who cannot be vaccinated due to medical reasons. 群体免疫保护因医学原因无法接种疫苗的人群。

10. Monoclonal Antibodies and Immunodeficiency | 单克隆抗体与免疫缺陷

Monoclonal antibodies are identical antibodies produced by a single clone of hybridoma cells. To produce them, a mouse is immunised with the antigen, and B lymphocytes from its spleen are fused with myeloma cells. The resulting hybridoma cells can be cultured indefinitely and secrete one specific antibody type.

单克隆抗体是由单个杂交瘤细胞克隆产生的相同抗体。制备时，先用抗原免疫小鼠，然后将其脾脏中的 B 淋巴细胞与骨髓瘤细胞融合。所得的杂交瘤细胞可无限培养，并分泌同一种特异性抗体。

Monoclonal antibodies have many applications. They can be used in diagnostic tests, such as pregnancy tests and home lateral-flow antigen tests for infectious diseases. They can also be used for targeted drug delivery: by attaching a cytotoxic drug to a monoclonal antibody that recognises a cancer antigen, the drug is delivered selectively to cancer cells, reducing damage to healthy tissue.

单克隆抗体用途广泛。它们可用于诊断检测，如妊娠检测和家用侧向流动抗原检测。也可用于靶向药物递送：将细胞毒性药物连接在识别癌抗原的单克隆抗体上，使药物选择性到达癌细胞，减少对健康组织的损伤。

Immunodeficiency occurs when one or more components of the immune system are defective. HIV is a well-known example; it mainly infects and destroys CD4⁺ helper T cells. As helper T cell numbers drop, the body cannot coordinate effective immune responses, leading to opportunistic infections that are normally controlled by a healthy immune system. AIDS is the final stage of HIV infection.

免疫缺陷是指免疫系统的一个或多个组成部分发生功能异常。HIV 是一个众所周知的例子，它主要感染并破坏 CD4⁺ 辅助 T 细胞。随着辅助 T 细胞数量下降，机体无法协调有效的免疫应答，导致正常健康免疫系统能够控制的机会性感染发生。艾滋病是 HIV 感染的终末期。

Understanding HIV has also helped scientists clarify the central role of helper T cells in both humoral and cell-mediated immunity. This reinforces why the immune system must be viewed as an interconnected network rather than a set of isolated defences.

对 HIV 的研究也帮助科学家明确了辅助 T 细胞在体液免疫和细胞免疫中的核心作用。这再次说明，免疫系统应当被视为一个相互连接的网络，而非一组孤立的防御机制。

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Human Defence Mechanisms Against Disease | 人体防御疾病的免疫机制