📚 Active and Passive Immunity | 主动免疫与被动免疫
The immune system defends the body against pathogens through a complex network of cells and molecules. While innate immunity provides rapid, non‑specific defence, adaptive immunity offers highly specific protection. Adaptive immunity can be acquired in two fundamentally different ways: active immunity, where the body generates its own immune response and memory, and passive immunity, where ready‑made antibodies are transferred from an external source.
免疫系统通过复杂的细胞和分子网络保护机体免受病原体侵害。先天免疫提供快速、非特异性的防御，而适应性免疫则提供高度特异的保护。获得适应性免疫有两种截然不同的方式：主动免疫——机体自身产生免疫应答和记忆；被动免疫——从外源获取现成的抗体。
1. Immunity: An Overview | 免疫概述
The immune response is broadly divided into innate (non‑specific) and adaptive (specific) branches. Adaptive immunity specifically recognises antigens and generates immunological memory. This memory is the hallmark of active immunity, but it is absent in passive immunity. Understanding the distinction between active and passive immunity is fundamental to appreciating vaccination, maternal antibody transfer, and immunoglobulin therapy.
免疫应答可大致分为先天免疫（非特异性）和适应性免疫（特异性）。适应性免疫能特异性识别抗原并建立免疫记忆。这种记忆是主动免疫的标志，但被动免疫则缺乏记忆。理解主动免疫与被动免疫的区别是认识疫苗接种、母体抗体传递和免疫球蛋白疗法的基础。
2. What is Active Immunity? | 什么是主动免疫？
Active immunity arises when the immune system encounters an antigen and mounts its own adaptive response. B lymphocytes differentiate into plasma cells that secrete antibodies, and memory B and T cells are generated. Because the body manufactures its own effectors and memory cells, the protection is long‑lasting, often persisting for years or even a lifetime.
主动免疫在免疫系统遭遇抗原并产生自身适应性应答时产生。B淋巴细胞分化为浆细胞分泌抗体，同时生成记忆B细胞和记忆T细胞。由于机体自行制造效应细胞和记忆细胞，这种保护作用持久，通常可维持数年乃至终身。
3. Natural Active Immunity | 自然主动免疫
Natural active immunity is acquired when a person recovers from an infection. During the first exposure, the primary immune response is relatively slow, and the individual may develop symptoms. However, after the pathogen is cleared, memory lymphocytes remain in the circulation and lymphoid tissues.
自然主动免疫是在一个人从感染中康复后获得的。初次暴露时，初级免疫应答相对缓慢，个体可能出现症状。但病原体被清除后，记忆淋巴细胞会留在循环系统和淋巴组织中。
Should the same pathogen invade again, memory cells trigger a rapid, vigorous secondary response that usually prevents disease. Classic examples include measles and chickenpox; surviving these illnesses typically grants life‑long natural active immunity.
如果同一病原体再次侵袭，记忆细胞会引发快速、强烈的二次应答，通常能避免发病。典型例子如麻疹和水痘；从这些疾病中康复通常可赋予终身的自然主动免疫。
4. Artificial Active Immunity and Vaccines | 人工主动免疫与疫苗
Artificial active immunity is achieved through vaccination. A vaccine contains antigens derived from a pathogen but is designed not to cause the disease. The immune system recognises the foreign antigens and generates both antibodies and memory cells, providing protection similar to that of natural active immunity.
人工主动免疫通过疫苗接种实现。疫苗含有源自病原体的抗原，但设计成不致疾病。免疫系统识别外来抗原，产生抗体和记忆细胞，提供与自然主动免疫类似的保护。
Vaccines exist in several forms: live attenuated (e.g. MMR, yellow fever), inactivated (e.g. polio, hepatitis A), subunit or conjugate (e.g. hepatitis B, HPV), and toxoid (e.g. diphtheria, tetanus). Each form safely presents key antigens to prime the adaptive response without causing significant illness. Booster doses are often needed to maintain protective levels of memory cells.
疫苗有几种形式：减毒活疫苗（如MMR、黄热病疫苗）、灭活疫苗（如脊髓灰质炎、甲型肝炎）、亚单位或结合疫苗（如乙型肝炎、HPV）以及类毒素疫苗（如白喉、破伤风）。每种形式都能安全地呈递关键抗原以启动适应性应答，而不会导致严重疾病。通常需要加强剂次以维持有保护水平的记忆细胞。
The success of vaccination is demonstrated by the eradication of smallpox and the dramatic reduction in the incidence of polio, measles and many other infectious diseases worldwide.
疫苗接种的成功体现在天花的根除以及脊髓灰质炎、麻疹等多种传染病在全球发病率的大幅下降。
5. What is Passive Immunity? | 什么是被动免疫？
Passive immunity involves the transfer of pre‑formed antibodies from one individual to another. The recipient's immune system is not stimulated to produce its own antibodies or memory cells. Protection is immediate because the antibodies can neutralise pathogens straight away, but it is temporary, typically lasting only weeks to a few months.
被动免疫涉及将已生成的抗体从一个个体转移到另一个个体。受者免疫系统不会被刺激去产生自身的抗体或记忆细胞。由于抗体能立即中和病原体，保护是立竿见影的，但也是暂时的，通常只持续数周至几个月。
Since no immunological memory is established, passive immunity offers no long‑term defence against future encounters with the same pathogen.
因为没有建立免疫记忆，被动免疫无法为日后遭遇同一病原体提供长期防御。
6. Natural Passive Immunity | 自然被动免疫
Natural passive immunity occurs when maternal antibodies are transferred to the offspring. During pregnancy, IgG antibodies cross the placenta, protecting the foetus and newborn against infections to which the mother has immunity.
自然被动免疫发生于母体抗体传递给后代时。孕期IgG抗体穿过胎盘，保护胎儿和新生儿免受母亲已获得免疫的那些感染。
After birth, breastfeeding continues to supply protective antibodies. Colostrum and breast milk are rich in secretory IgA, which coats the infant's intestinal and respiratory mucosa, reducing the risk of pathogen entry. Over time, the transferred antibodies are degraded, and the infant's own adaptive immune system matures.
出生后，母乳喂养可持续提供保护性抗体。初乳和母乳富含分泌型IgA，可覆盖婴儿肠道和呼吸道黏膜，降低病原体入侵风险。随着时间的推移，转移的抗体被降解，婴儿自身的适应性免疫系统逐渐成熟。
7. Artificial Passive Immunity | 人工被动免疫
Artificial passive immunity results from the injection of antibodies obtained from another human or an animal. It is used when immediate protection is essential, such as after exposure to a toxin or a fast‑acting pathogen, or for immunocompromised patients who cannot mount an adequate active response.
人工被动免疫源于注射从其他人或动物获得的抗体。当需要即刻保护时，例如接触毒素或作用迅速的病原体后，或对于无法产生充分主动应答的免疫受损患者，采用该方法。
Examples include antivenom against snake bites, human rabies immunoglobulin given after a bite from a rabid animal, and anti‑tetanus immunoglobulin for tetanus‑prone wounds. These preparations neutralise the threat quickly while the patient's active response or other treatments take effect. If antibodies are derived from a non‑human source, there is a risk of serum sickness, but modern humanised monoclonal antibodies minimise this.
例子包括抗蛇毒血清、被患狂犬病动物咬伤后注射的人狂犬病免疫球蛋白，以及用于易患破伤风伤口的破伤风免疫球蛋白。这些制剂能迅速中和威胁，同时等待患者的主动应答或其他治疗生效。如果抗体来源于非人类物种，有血清病的风险，但现代人源化单克隆抗体已将这一风险降至最低。
8. Comparing Active and Passive Immunity | 主动免疫与被动免疫的比较
The table below highlights the key differences between active and passive immunity, encompassing origin of antibodies, speed of onset, duration and the generation of memory cells.
下表突显了主动免疫与被动免疫的关键区别，涵盖抗体来源、起效速度、持续时间以及记忆细胞的产生。
| Feature 特征 | Active Immunity 主动免疫 | Passive Immunity 被动免疫 |
|---|---|---|
| Origin of antibodies 抗体来源 | Produced by the body's own plasma cells 由身体自身的浆细胞产生 | Transferred from an external source 从外源转移而来 |
| Onset of protection 保护开始时间 | Slow; days to weeks 缓慢；数天至数周 | Immediate 即刻 |
| Duration 持续时间 | Long-term; years to lifelong 长期；数年至终身 | Short-term; weeks to months 短期；数周至数月 |
| Memory cells 记忆细胞 | Formed 产生 | Not formed 不产生 |
| Response to booster doses 对加强剂次的应答 | Effective; enhances memory 有效；可增强记忆 | Not applicable; no memory to boost 不适用；无记忆可增强 |
| Natural example 自然例子 | Recovery from infection 感染后康复 | Maternal antibodies via placenta and milk 通过胎盘和母乳获得母体抗体 |
| Artificial example 人工例子 | Vaccination 疫苗接种 | Injection of immunoglobulins 注射免疫球蛋白 |
9. The Importance of Memory Cells | 记忆细胞
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