📚 Active and Passive Immunity | 主动免疫与被动免疫
The immune system defends the body against pathogens through two broad strategies: active immunity, in which the body produces its own antibodies and memory cells, and passive immunity, in which ready-made antibodies are supplied from an external source. Understanding the differences between these forms of immunity is essential for explaining vaccination, maternal protection, and antiserum treatments.
免疫系统通过两大策略保护身体免受病原体侵害：主动免疫是由身体自身产生抗体和记忆细胞，而被动免疫则是从外部获得现成的抗体。理解这两种免疫形式的差异，对于解释疫苗接种、母体保护和抗血清治疗至关重要。
1. Defining Immunity | 免疫的定义
Immunity is the ability of an organism to resist infection by a particular pathogen. It can be innate (non-specific) or adaptive (specific), but in the context of active and passive immunity, we focus on adaptive immunity involving antibodies and lymphocytes.
免疫是指生物体抵抗特定病原体感染的能力。它可以分为先天性（非特异性）和适应性（特异性）免疫，但在主动免疫与被动免疫的语境中，我们主要关注涉及抗体和淋巴细胞的适应性免疫。
2. Overview of Active and Passive Immunity | 主动免疫与被动免疫概述
Active immunity occurs when the immune system is exposed to an antigen and responds by producing antibodies and memory cells. Passive immunity occurs when antibodies are transferred from one individual to another, providing immediate but temporary protection without memory cell formation.
主动免疫发生在免疫系统接触抗原并作出反应，产生抗体和记忆细胞时。被动免疫则是抗体从一个个体转移到另一个个体，提供即时但暂时的保护，且不形成记忆细胞。
3. Natural Active Immunity | 天然主动免疫
Natural active immunity develops when a person is infected by a pathogen. The immune system recognises the pathogen's antigens, produces specific antibodies, and generates memory B and T cells. This often gives long-lasting protection against that pathogen.
天然主动免疫在人体感染病原体后形成。免疫系统识别病原体的抗原，产生特异性抗体，并生成记忆 B 细胞和 T 细胞。这通常能对该病原体提供持久的保护。
If the same pathogen enters the body again, memory cells rapidly divide and differentiate into plasma cells, producing a faster and stronger secondary response. This is why many diseases, such as measles, usually do not occur twice in the same individual.
如果同一种病原体再次进入体内，记忆细胞会迅速分裂并分化为浆细胞，产生更快更强的二次应答。这就是为什么许多疾病（如麻疹）通常不会在同一个人身上发生两次。
4. Artificial Active Immunity: Vaccination | 人工主动免疫：疫苗接种
Artificial active immunity is achieved through vaccination. A vaccine contains antigens from a pathogen, or weakened/inactivated whole pathogens, or mRNA instructions for an antigen. The body mounts an immune response, producing antibodies and memory cells without causing severe disease.
人工主动免疫通过接种疫苗实现。疫苗含有来自病原体的抗原、减毒或灭活的完整病原体，或编码抗原的 mRNA 指令。身体产生免疫应答，生成抗体和记忆细胞，而不会引起严重疾病。
Because memory cells persist, vaccination provides long-term protection. Booster doses may be needed for some vaccines to maintain antibody levels or to deal with antigenic variation.
由于记忆细胞持续存在，疫苗接种可提供长期保护。某些疫苗可能需要加强针以维持抗体水平或应对抗原变异。
5. Natural Passive Immunity | 天然被动免疫
Natural passive immunity occurs when antibodies pass from mother to child. During pregnancy, IgG antibodies cross the placenta into the fetal bloodstream. After birth, secretory IgA antibodies are also transferred in breast milk, particularly in colostrum.
天然被动免疫发生在抗体从母亲传递给婴儿时。怀孕期间，IgG 抗体穿过胎盘进入胎儿血液循环。出生后，分泌型 IgA 抗体也通过母乳（尤其是初乳）传递。
These antibodies give the newborn immediate protection against common pathogens for the first few months of life. However, because the infant's immune system does not make its own memory cells, this protection is temporary and fades as the antibodies are broken down.
这些抗体为新生儿在生命最初几个月提供即时的保护，抵御常见病原体。但由于婴儿的免疫系统并未产生自身的记忆细胞，这种保护是暂时的，并会随着抗体被分解而消失。
6. Artificial Passive Immunity | 人工被动免疫
Artificial passive immunity involves injecting antibodies obtained from another source. These antibodies may be collected from human donors (pooled immunoglobulins) or from animals such as horses (antiserum). It gives immediate protection but lasts only a few weeks or months.
人工被动免疫涉及注射从其他来源获得的抗体。这些抗体可以从人类供体（混合免疫球蛋白）或马等动物（抗血清）中收集。它能提供即时保护，但仅持续数周或数月。
Examples include tetanus antiserum after a deep wound, rabies immunoglobulin after a bite, and anti-venom for snake bites. This approach is useful when there is no time for the patient to develop an active immune response.
实例包括深伤口后注射破伤风抗血清、被咬伤后注射狂犬病免疫球蛋白，以及蛇咬伤时使用抗蛇毒血清。当患者没有时间产生主动免疫应答时，这种方法非常有用。
7. Comparing Active and Passive Immunity | 主动免疫与被动免疫比较
The table below summarises the key differences between active and passive immunity.
下表总结了主动免疫与被动免疫之间的主要区别。
| Feature | Active immunity | Passive immunity |
|---|---|---|
| Source of antibodies | Produced by the person's own plasma cells | Introduced from an external source |
| Onset of protection | Slow - takes days to weeks | Immediate |
| Duration | Long-lasting, often years or lifelong | Short-lived, weeks to months |
| Memory cells | Formed | Not formed |
| Examples | Infection, vaccination | Placental transfer, breast milk, antiserum |
In an exam, it is important to use the terms 'active' and 'passive' accurately. Active immunity involves the body's own immune response; passive immunity does not.
在考试中，准确使用“主动”和“被动”这两个术语很重要。主动免疫涉及身体自身的免疫应答；被动免疫则不然。
8. Antibodies and Antigens in Immunity | 免疫中的抗体与抗原
Antigens are molecules, usually proteins or polysaccharides on pathogen surfaces, that trigger an immune response. Antibodies are Y-shaped glycoproteins (immunoglobulins) produced by plasma cells. Each antibody has a variable region that binds specifically to one antigen.
抗原是触发免疫应答的分子，通常是病原体表面的蛋白质或多糖。抗体是由浆细胞产生的 Y 形糖蛋白（免疫球蛋白）。每种抗体都有一个可变区，能特异性地结合某一种抗原。
In active immunity, the body produces antibodies that match the antigen's shape. In passive immunity, the injected or transferred antibodies already match the antigen, so they can neutralise pathogens or toxins immediately.
在主动免疫中，身体产生与抗原形状相匹配的抗体。在被动免疫中，注射或转移的抗体已经与抗原匹配，因此能够立即中和病原体或毒素。
9. Memory Cells and the Secondary Response | 记忆细胞与二次应答
Memory B and T cells are produced during a primary immune response. They remain in the body for a long time. Upon re-exposure to the same antigen, memory cells respond more rapidly and secrete more antibodies than the primary response.
记忆 B 细胞和 T 细胞在初次免疫应答过程中产生。它们在体内长期存在。当再次接触相同抗原时，记忆细胞的应答速度更快，分泌的抗体也比初次应答更多。
The graph of antibody concentration over time shows a longer lag phase in the primary response and a rapid, higher peak in the secondary response. Active immunity depends on this memory; passive immunity does not generate memory cells.
抗体浓度随时间变化的曲线显示，初次应答的滞后期较长，二次应答则迅速达到更高的峰值。主动免疫依赖这种记忆；被动免疫不会产生记忆细胞。
10. Herd Immunity | 群体免疫
Herd immunity occurs when a sufficiently high proportion of a population is immune to a pathogen, making it difficult for the pathogen to spread. Vaccination is the safest way to achieve herd immunity because it generates active immunity without causing disease.
当人群中足够高比例的人对某种病原体具有免疫力时，病原体难以传播，这就是群体免疫。疫苗接种是获得群体免疫最安全的方式，因为它能产生主动免疫而不引起疾病。
Passive immunity does not contribute significantly to herd immunity because it is temporary and does not create memory cells in the protected individual.
被动免疫对群体免疫的贡献不大，因为它是暂时的，并且不会在受保护者体内产生记忆细胞。
11. Examples of Passive Immunity in Medicine | 被动免疫的医学实例
Several clinical situations use passive immunity. Tetanus immunoglobulin is given after a deep or dirty wound if the patient's vaccination status is uncertain. Rabies immunoglobulin is given after exposure to a rabid animal, alongside the rabies vaccine.
有几种临床情况会使用被动免疫。如果患者的破伤风疫苗接种情况不明，在深或脏的伤口后会给予破伤风免疫球蛋白。暴露于患狂犬病的动物后，除了接种狂犬病疫苗外，还会给予狂犬病免疫球蛋白。
Anti-venom for snake bites and botulinum antitoxin are other examples. In each case, the passive antibodies neutralise the toxin or pathogen before the patient's active immune response can develop.
抗蛇毒血清和肉毒杆菌抗毒素是其他例子。在每种情况下，被动抗体在患者的主动免疫应答形成之前就中和了毒素或病原体。
12. Exam-Style Summary | 考试要点总结
ng duration; passive = external antibodies + no memory cells + immediate + short duration.
在剑桥 A-Level 生物考试中，你可能会被要求比较主动免疫和被动免疫、解释疫苗的工作原理，或评价抗血清的使用。需要记住的要点：主动 = 自身抗体 + 记忆细胞 + 起效慢 + 持续时间长；被动 = 外部抗体 + 无记忆细胞 + 即时 + 持续时间短。
Always link the presence or absence of memory cells to the duration of protection, and use precise terminology such as 'plasma cells', 'memory B cells', 'IgG', and 'antigen' in longer answers.
在较长答案中，始终将记忆细胞的有无与保护持续时间联系起来，并使用精确的术语，如“浆细胞”、“记忆 B 细胞”、“IgG”和“抗原”。
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