📚 Active Immunity vs Passive Immunity | 主动免疫与被动免疫的区别
Immunity is the body’s ability to recognise and defend itself against pathogens. In A-Level biology, you must distinguish between active and passive immunity, and understand the mechanisms, duration, and medical applications of each type.
免疫是身体识别并防御病原体的能力。在 A-Level 生物学中，你必须区分主动免疫与被动免疫，并理解每种类型的机制、持续时间及医学应用。
1. What Is Immunity? | 什么是免疫？
Immunity can be defined as resistance to infection, usually conferred by the immune system. It involves both non-specific (innate) and specific (adaptive) defences. Adaptive immunity is highly specific and provides long-lasting protection through memory cells.
免疫可定义为对感染的抵抗力，通常由免疫系统赋予。它涉及非特异性（固有）防御和特异性（适应性）防御。适应性免疫具有高度特异性，并通过记忆细胞提供长期保护。
Adaptive immunity is further classified into two broad categories: active immunity and passive immunity. The critical distinction is whether the body produces its own antibodies or receives ready-made antibodies from an external source.
适应性免疫进一步分为两大类：主动免疫和被动免疫。关键区别在于身体是自行产生抗体，还是从外部来源获得现成抗体。
2. Active Immunity | 主动免疫
Active immunity occurs when the body’s own immune system is stimulated to produce antibodies and memory cells. This requires the presence of an antigen, which may be introduced naturally through infection or artificially through vaccination.
主动免疫是指人体自身的免疫系统受到刺激而产生抗体和记忆细胞。这个过程需要抗原的存在，抗原可以通过自然感染或人工接种疫苗的方式进入人体。
Because the body actively participates in antibody production, active immunity takes time to develop — usually several days to weeks. However, once established, it is long-lasting and often life-long because memory cells remain in the body.
由于身体积极参与抗体产生，主动免疫需要时间才能形成——通常为数天至数周。然而，一旦建立，它就是持久的，通常是终身的，因为记忆细胞会留在体内。
Active immunity is characterised by two features: the production of antibodies by plasma cells, and the formation of memory B and T cells that enable a rapid secondary response.
主动免疫有两个特征：浆细胞产生抗体，以及形成记忆 B 细胞和记忆 T 细胞以确保快速的二次应答。
3. Passive Immunity | 被动免疫
Passive immunity involves the transfer of antibodies from one individual to another. In this case, the recipient does not produce antibodies or memory cells; instead, they receive ready-made antibodies from an external source.
被动免疫是指抗体从一个个体转移到另一个个体。在这种情况下，接受者不会自己产生抗体或记忆细胞；相反，他们从外部来源获得现成的抗体。
This type of immunity provides immediate protection because no time is needed for antibody synthesis. However, it is temporary — it lasts only for weeks or months, because the transferred antibodies are naturally degraded and no memory cells are formed.
这种免疫提供即时保护，因为不需要时间合成抗体。然而，它是暂时的——只能持续数周或数月，因为转移的抗体自然降解，而且没有形成记忆细胞。
Passive immunity can occur naturally, such as IgG antibodies crossing the placenta, or artificially, such as administering antiserum to a patient with tetanus.
被动免疫可以自然发生，例如 IgG 抗体通过胎盘传递给胎儿；也可以人工发生，例如给破伤风患者注射抗血清。
4. Key Differences at a Glance | 主要区别一览
The table below summarises the essential differences between active and passive immunity, which you should be prepared to compare in an exam.
下表总结了主动免疫与被动免疫之间本质上的区别，你应该准备好在考试中对它们进行比较。
| Feature | Active Immunity | Passive Immunity |
| Antibody production | Produced by the individual’s own plasma cells | Received from an external source |
| Memory cells | Yes — formed | No — not formed |
| Onset | Slow (days to weeks) | Immediate |
| Duration | Long-lasting (often years or lifetime) | Short-lived (weeks to months) |
| Natural example | Recovery from infection | Maternal antibodies across placenta |
| Artificial example | Vaccination | Immunoglobulin injection |
5. Natural Active Immunity | 天然主动免疫
Natural active immunity arises when a person naturally encounters a pathogen and mounts an immune response. For example, if you recover from chickenpox, your body has produced antibodies and memory cells against the varicella-zoster virus.
天然主动免疫是指一个人自然接触病原体并产生免疫应答。例如，如果你从水痘中康复，你的身体就产生了针对水痘-带状疱疹病毒的抗体和记忆细胞。
The primary response to infection is relatively slow because, on first exposure, the immune system must select and clone appropriate B lymphocytes. Eventually, plasma cells secrete enough antibody to clear the infection, and memory cells remain for decades.
初次感染时的初次应答相对较慢，因为在首次接触时，免疫系统必须选择和克隆合适的 B 淋巴细胞。最终，浆细胞分泌足量抗体以清除感染，而记忆细胞可留存数十年。
It is important to note that natural active immunity may result in illness, which is a risk. Nevertheless, it produces robust and comprehensive immunity, often including both humoral and cell-mediated components.
重要的是要注意，天然主动免疫可能导致疾病，这是一个风险。尽管如此，它能产生强大而全面的免疫力，通常同时包含体液免疫和细胞免疫成分。
6. Artificial Active Immunity | 人工主动免疫（疫苗接种）
Artificial active immunity is induced by vaccination. A vaccine contains an antigen — either a weakened (attenuated) pathogen, an inactivated pathogen, a toxin (toxoid), or a subunit of a pathogen such as a surface protein.
人工主动免疫通过接种疫苗诱导产生。疫苗含有抗原——可以是减毒病原体、灭活病原体、类毒素（脱毒毒素），或病原体亚单位（如表面蛋白）。
When the vaccine is administered, the immune system responds in the same way as it would to a natural infection: B cells are activated, antibodies are produced, and memory cells are generated. But because the vaccine does not cause full disease, the risk is greatly reduced.
接种疫苗后，免疫系统以与自然感染相同的方式作出应答：B 细胞被激活，产生抗体，并生成记忆细胞。但由于疫苗不会引起完整疾病，风险大大降低。
Booster vaccinations are sometimes needed because antibody levels may decline over time. A booster rapidly reactivates memory cells, producing a strong secondary response and prolonging immunity.
有时需要加强免疫接种，因为抗体水平可能随时间下降。加强剂能快速重新激活记忆细胞，产生强大的二次应答，并延长免疫力。
7. Natural Passive Immunity | 天然被动免疫
Natural passive immunity occurs when antibodies are transferred from mother to offspring. During pregnancy, IgG antibodies cross the placenta via Fc receptors, providing the fetus with maternal antibodies.
天然被动免疫发生在抗体从母体传递给后代时。怀孕期间，IgG 抗体通过 Fc 受体穿过胎盘，为胎儿提供母源抗体。
After birth, IgA antibodies are also passed to the infant through breast milk, especially in colostrum. These antibodies protect the infant’s gut and respiratory tract during the first few months of life.
出生后，IgA 抗体也通过母乳（尤其是初乳）传给婴儿。这些抗体在婴儿出生后的头几个月里保护其肠道和呼吸道。
This passive immunity is temporary. The maternal antibodies gradually break down, and the infant’s own immune system becomes increasingly responsible for defence. By around 6–12 months, most maternal antibodies have disappeared.
这种被动免疫是暂时的。母源抗体逐渐降解，婴儿自身的免疫系统逐渐承担起防御责任。到 6–12 个月大时，大部分母源抗体已消失。
8. Artificial Passive Immunity | 人工被动免疫（免疫球蛋白疗法）
Artificial passive immunity involves injecting ready-made antibodies — known as immunoglobulins or antiserum — into a patient. This is used for immediate protection after exposure to a dangerous pathogen, such as in tetanus, rabies, or hepatitis B.
人工被动免疫是指向患者注射现成抗体——称为免疫球蛋白或抗血清。这用于暴露于危险病原体后的即时保护，例如破伤风、狂犬病或乙型肝炎。
For example, if a person with no tetanus vaccination sustains a deep, dirty wound, they may receive tetanus immunoglobulin (anti-toxin) immediately. The antibodies neutralise the tetanus toxin while natural active immunity may be induced simultaneously by vaccination.
例如，如果未接种破伤风疫苗的人受到深而脏的伤口，他们可能立即注射破伤风免疫球蛋白（抗毒素）。抗体中和破伤风毒素，同时可能通过接种疫苗诱导天然主动免疫。
Artificial passive immunity is also used in the treatment of snake venom poisoning, where specific antivenom antibodies are given as quickly as possible. It is also used for immunocompromised patients who cannot make antibodies effectively.
人工被动免疫也用于治疗蛇毒中毒，尽可能快地给予特异性抗蛇毒抗体。它也可用于无法有效产生抗体的免疫功能低下患者。
9. Memory Response: Why Active Immunity Lasts | 记忆反应：为什么主动免疫更持久
The key to the long duration of active immunity lies in memory cells. After an initial infection or vaccination, memory B cells and memory T cells remain in the body, often for years or even a lifetime.
主动免疫持续时间长的关键在于记忆细胞。在初次感染或接种疫苗后，记忆 B 细胞和记忆 T 细胞留在体内，通常能维持数年甚至终身。
If the same antigen enters the body again, these memory cells divide rapidly and differentiate into plasma cells and effector T cells. This secondary response is faster, larger, and more effective than the primary response.
如果相同的抗原再次进入体内，这些记忆细胞会迅速分裂并分化为浆细胞和效应 T 细胞。这种二次应答比初次应答更快、更强、更有效。
Because passive immunity does not generate memory cells, the protection it offers is transient. Once the transferred antibodies are removed by the body, the individual becomes susceptible again.
由于被动免疫不产生记忆细胞，它提供的保护是短暂的。一旦转运的抗体被身体清除，个体将再次变得易感。
10. Advantages and Disadvantages | 优势与劣势
Both types of immunity have clear clinical roles, and understanding their trade-offs is essential for evaluating public health strategies.
两种免疫类型都有明确的临床用途，理解它们之间的权衡对评估公共卫生策略至关重要。
Active immunity advantages:
主动免疫的优势：
- Long-lasting protection due to memory cells
- Used for vaccines to control epidemics
- No risk of disease when using modern vaccines
- 由于记忆细胞提供长期保护
- 用于疫苗以控制流行病
- 使用现代疫苗时没有患病风险
Active immunity disadvantages:
主动免疫的劣势：
- Slow onset — needs days or weeks to develop
- Requires multiple doses or boosters for some vaccines
- May fail in immunocompromised individuals
- 起效缓慢——需要数天或数周才能形成
- 某些疫苗需要多剂或加强针
- 在免疫功能低下者中可能无效
Passive immunity advantages:
被动免疫的优势：
- Immediate protection for post-exposure prophylaxis
- Useful for patients who cannot produce antibodies
- Provides toxin neutralisation quickly
- 为暴露后预防提供即时保护
- 适用于不能产生抗体的患者
- 快速中和毒素
Passive immunity disadvantages:
被动免疫的劣势：
- Short duration — no memory cells
- Risk of allergic reactions (serum sickness)
- Requires repeated injections for ongoing protection
- 持续时间短——无记忆细胞
- 有过敏反应风险（血清病）
- 需要反复注射以持续保护
11. Clinical Applications and Exam Focus | 临床应用与考点重点
In CIE A-Level biology, you may be asked to interpret a scenario and decide whether immunity is active or passive, natural or artificial. Remember the four-way classification: natural active, artificial active, natural passive, and artificial passive.
在 CIE A-Level 生物学中，你可能会被要求解读一个情景，并判断免疫力是主动还是被动、天然还是人工。记住四种分类：天然主动、人工主动、天然被动和人工被动。
A common example is vaccination against tetanus, which is artificial active immunity. But treatment of tetanus with antitoxin is artificial passive immunity. A baby receiving antibodies through breast milk is natural passive immunity.
一个常见例子是接种破伤风疫苗，这属于人工主动免疫。而用抗毒素治疗破伤风则属于人工被动免疫。婴儿通过母乳获得抗体属于天然被动免疫。
Be careful with exam questions that describe “memory cells” or “long-term immunity” — these point to active immunity. Questions that mention “immediate protection” or “injection of antibodies” point to passive immunity.
要注意考试中描述“记忆细胞”或“长期免疫”的题目——这些指向主动免疫。提及“即时保护”或“注射抗体”的题目指向被动免疫。
12. Summary | 总结
Active immunity is the production of antibodies and memory cells by the individual’s own immune system, offering long-lasting protection. It can be natural, through infection, or artificial, through vaccination.
主动免疫是个体自身免疫系统产生抗体和记忆细胞，提供长期保护。它可以通过自然感染获得（天然），也可以通过疫苗接种获得（人工）。
Passive immunity is the transfer of ready-made antibodies from another source, giving immediate but temporary protection. It can be natural, via maternal antibodies, or artificial, via immunoglobulin injections.
被动免疫是从外部来源获得现成抗体，提供即时但暂时的保护。它可以通过母源抗体自然获得，也可以通过注射免疫球蛋白人工获得。
The distinction is one of the most frequently tested concepts in A-Level biology. Mastering it will help you answer both multiple-choice questions and longer essay-style questions with confidence.
这个区别是 A-Level 生物学中最常考的概念之一。掌握它有助于你自信地回答选择题和较长的论述题。
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