📚 Acquired Immune Deficiency Syndrome (AIDS) | 获得性免疫缺陷综合征（艾滋病）
Acquired Immune Deficiency Syndrome, commonly known as AIDS, is a life‑threatening condition caused by the human immunodeficiency virus (HIV). It represents the most advanced stage of HIV infection, characterised by a severely weakened immune system that can no longer defend the body against opportunistic infections and certain cancers. Understanding the biology of HIV, its mode of action on T‑helper lymphocytes, and the progression to AIDS is central to the A‑Level Biology syllabus.
获得性免疫缺陷综合征，即人们常说的艾滋病，是由人类免疫缺陷病毒（HIV）引起的一种危及生命的疾病。它是HIV感染的最晚期阶段，以免疫系统严重受损为特征，此时身体已无法抵御机会性感染和某些癌症。理解HIV的生物学特性、其对T辅助淋巴细胞的作用机制以及从感染到发展为艾滋病的过程，是A‑Level生物教学大纲的核心内容。
1. What is AIDS? | 什么是艾滋病？
AIDS is not a single disease but a syndrome – a collection of signs and symptoms that result from the destruction of the immune system by HIV. A person is diagnosed with AIDS when their CD4⁺ T‑helper cell count falls below 200 cells per microlitre of blood, or when they develop one or more specific opportunistic infections or cancers, regardless of the CD4 count. The syndrome highlights the critical importance of T‑helper lymphocytes in orchestrating the adaptive immune response.
艾滋病不是一种单一的疾病，而是一种综合征——即由HIV摧毁免疫系统而引起的一系列体征和症状的集合。当个体的CD4⁺ T辅助细胞计数降至每微升血液200个以下，或者无论CD4计数如何，只要出现一种或多种特定的机会性感染或癌症时，即可诊断为艾滋病。这一综合征凸显了T辅助淋巴细胞在协调适应性免疫应答中的关键作用。
2. The Causative Agent: HIV | 病原体：人类免疫缺陷病毒（HIV）
Human immunodeficiency virus (HIV) is a retrovirus belonging to the lentivirus genus. There are two main types: HIV‑1, which is responsible for the global pandemic, and HIV‑2, which is less pathogenic and largely confined to West Africa. As a retrovirus, HIV carries its genetic information as single‑stranded RNA and uses the enzyme reverse transcriptase to convert RNA into DNA once inside the host cell, enabling integration into the host genome.
人类免疫缺陷病毒（HIV）是一种属于慢病毒属的逆转录病毒。主要有两种类型：HIV‑1是全球大流行的元凶，HIV‑2致病性较低，主要局限于西非地区。作为一种逆转录病毒，HIV以单链RNA的形式携带遗传信息，并利用逆转录酶在进入宿主细胞后将RNA转化为DNA，从而使其能够整合到宿主基因组中。
3. Structure of HIV | HIV的结构
The HIV virion is spherical, approximately 100–120 nm in diameter. Its structure includes a lipid envelope derived from the host cell membrane, studded with viral glycoproteins gp120 and gp41. Beneath the envelope lies the matrix protein layer and the capsid (core), which encloses two copies of single‑stranded RNA, along with essential enzymes: reverse transcriptase, integrase, and protease. The glycoprotein gp120 is responsible for binding to the CD4 receptor on T‑helper cells, the critical first step in infection.
HIV病毒颗粒呈球形，直径约100–120纳米。其结构包括一层来自宿主细胞膜的脂质包膜，包膜上嵌有病毒糖蛋白gp120和gp41。包膜下方是基质蛋白层和衣壳（核心），衣壳内包裹着两条单链RNA拷贝以及必需的酶：逆转录酶、整合酶和蛋白酶。糖蛋白gp120负责与T辅助细胞表面的CD4受体结合，这是感染的关键第一步。
4. HIV Replication Cycle | HIV的复制周期
The replication cycle of HIV can be broken down into several stages. First, the gp120 protein on the viral envelope attaches to the CD4 receptor and a co‑receptor (usually CCR5) on the surface of a host T‑helper lymphocyte. This binding triggers fusion of the viral envelope with the cell membrane, allowing the viral capsid to enter the cytoplasm. Once inside, the RNA genome is reverse‑transcribed into complementary DNA (cDNA) by reverse transcriptase. The cDNA then enters the nucleus and is integrated into the host chromosome by the enzyme integrase, forming a provirus. The host’s transcription machinery transcribes the proviral DNA into new viral RNA, which serves both as genomic material for new virions and as mRNA for viral protein synthesis. After translation, the viral RNA and proteins assemble at the cell membrane, and new virions bud off, acquiring their envelope from the host cell. Finally, the protease enzyme cleaves precursor polyproteins to produce mature, infectious virus particles.
HIV的复制周期可分为几个阶段。首先，病毒包膜上的gp120蛋白与宿主T辅助淋巴细胞表面的CD4受体及辅助受体（通常为CCR5）结合。这种结合触发了病毒包膜与细胞膜的融合，使病毒衣壳进入细胞质。进入后，RNA基因组被逆转录酶逆转录为互补DNA（cDNA）。cDNA随后进入细胞核，并由整合酶整合到宿主染色体中，形成前病毒。宿主的转录机构将前病毒DNA转录为新的病毒RNA，这些RNA既可作为新病毒颗粒的基因组材料，也可作为mRNA指导病毒蛋白的合成。翻译完成后，病毒RNA和蛋白质在细胞膜处组装，新的病毒颗粒以出芽方式释放，并在此过程中从宿主细胞获得包膜。最后，蛋白酶切割前体多聚蛋白，产生成熟且有感染性的病毒颗粒。
5. The Role of T‑helper Lymphocytes | T辅助淋巴细胞的作用
T‑helper cells (CD4⁺ lymphocytes) are central regulators of the immune system. They recognise antigens presented by antigen‑presenting cells and secrete cytokines that activate B cells to produce antibodies, stimulate cytotoxic T cells to kill infected cells, and enhance macrophage activity. By selectively infecting and destroying T‑helper cells, HIV dismantles the body’s capacity to mount both humoral and cell‑mediated immune responses. The gradual decline in CD4⁺ cells renders the host increasingly susceptible to pathogens that would normally be controlled with ease.
T辅助细胞（CD4⁺淋巴细胞）是免疫系统的核心调控者。它们识别由抗原呈递细胞呈递的抗原，并分泌细胞因子，从而激活B细胞产生抗体、刺激细胞毒性T细胞杀伤受感染细胞，并增强巨噬细胞的活性。通过选择性感染并摧毁T辅助细胞，HIV瓦解了机体发动体液免疫和细胞免疫应答的能力。CD4⁺细胞的逐步减少使宿主越来越容易受到那些通常本可被轻松控制的病原体的侵袭。
6. Progression from HIV Infection to AIDS | 从HIV感染发展到艾滋病
HIV infection progresses through several clinical stages. During the acute phase, within 2–4 weeks of exposure, many individuals experience flu‑like symptoms as the virus replicates rapidly and CD4⁺ cell numbers dip temporarily. The immune system responds by producing anti‑HIV antibodies and mounting a cellular response, which partially controls the virus. This is followed by a long clinical latency period (sometimes lasting up to 10 years or more) during which the virus continues to replicate at low levels and CD4⁺ cells slowly decline. Without treatment, the progressive loss of T‑helper cells eventually leads to severe immunosuppression. When the CD4⁺ count falls below 200 cells/μL or specific AIDS‑defining illnesses appear, the infection has progressed to AIDS.
HIV感染经历若干临床阶段。在急性期，即暴露后2‑4周内，许多个体会出现流感样症状，此时病毒迅速复制，CD4⁺细胞数量暂时下降。免疫系统随即产生抗HIV抗体并发动细胞免疫应答，从而部分控制病毒。随后进入漫长的临床潜伏期（有时可持续长达10年或更久），期间病毒以较低水平持续复制，CD4⁺细胞缓慢减少。若不进行治疗，T辅助细胞的持续丢失最终会导致严重的免疫抑制。当CD4⁺计数降至每微升200个以下，或出现特定的艾滋病指征性疾病时，感染便发展成了艾滋病。
7. Opportunistic Infections and Symptoms | 机会性感染与症状
With the collapse of the immune system, pathogens that are normally harmless or easily eliminated can cause life‑threatening illnesses. Common AIDS‑defining opportunistic infections include Pneumocystis pneumonia (caused by the fungus Pneumocystis jirovecii), tuberculosis (Mycobacterium tuberculosis), candidiasis of the oesophagus, and cytomegalovirus retinitis. Additionally, certain cancers such as Kaposi’s sarcoma (linked to human herpesvirus 8) and non‑Hodgkin lymphoma occur with much higher frequency. Clinical symptoms may include severe weight loss (wasting syndrome), persistent fever, night sweats, chronic diarrhoea, and swollen lymph nodes, reflecting the body’s inability to combat multiple pathogens simultaneously.
随着免疫系统的崩溃，那些通常无害或容易清除的病原体可引起危及生命的疾病。常见的艾滋病指征性机会性感染包括肺孢子菌肺炎（由真菌耶氏肺孢子菌引起）、结核病（结核分枝杆菌）、食管念珠菌病以及巨细胞病毒视网膜炎。此外，某些癌症如卡波西肉瘤（与人疱疹病毒8型相关）和非霍奇金淋巴瘤的发病率也显著增高。临床症状可能包括严重体重下降（消耗综合征）、持续发烧、盗汗、慢性腹泻和淋巴结肿大，这些都反映出身体已无力同时对抗多种病原体。
8. Transmission of HIV | HIV的传播途径
HIV is transmitted through direct contact with certain body fluids – primarily blood, semen, vaginal and rectal fluids, and breast milk – from an infected person. The main routes of transmission are unprotected sexual intercourse (vaginal, anal, or oral), sharing contaminated needles or syringes (common among injecting drug users), transfusion of infected blood or blood products (now extremely rare due to rigorous screening), and mother‑to‑child transmission during pregnancy, childbirth, or breastfeeding. Importantly, HIV is not spread through casual contact, such as hugging, shaking hands, or sharing utensils, nor through insect bites.
HIV通过直接接触感染者的特定体液传播，这些体液主要包括血液、精液、阴道和直肠分泌液以及母乳。主要传播途径为无保护的性交（阴道、肛门或口交）、共用被污染的针头或注射器（常见于注射吸毒者）、输入被感染的血液或血制品（如今因严格筛查已极为罕见），以及怀孕、分娩或母乳喂养期间的母婴传播。需要强调的是，HIV不会通过日常接触传播，如拥抱、握手、共用餐具，也不会通过昆虫叮咬传播。
9. Diagnosis of HIV Infection | HIV感染的诊断
HIV diagnosis is typically based on detecting antibodies against the virus or the virus’s own components. The most common initial test is the enzyme‑linked immunosorbent assay (ELISA), which detects anti‑HIV antibodies in a blood sample. Because it can take several weeks for antibodies to reach detectable levels (the ‘window period’), a fourth‑generation test that simultaneously detects p24 antigen (a viral capsid protein) and antibodies is now widely used to shorten this window. Positive ELISA results are confirmed by a more specific test, such as the Western blot or nucleic acid amplification tests (NAAT) that directly detect viral RNA. Rapid point‑of‑care tests are also available for on‑the‑spot screening.
HIV的诊断通常基于检测针对病毒的抗体或病毒本身的成分。最常见的初筛试验是酶联免疫吸附测定（ELISA），可检测血液样本中的抗HIV抗体。由于抗体达到可检测水平需要数周时间（“窗口期”），目前广泛使用的第四代检测法可同时检测p24抗原（一种病毒衣壳蛋白）和抗体，从而缩短窗口期。阳性ELISA结果需通过更特异的试验加以确认，例如蛋白质印迹法或直接检测病毒RNA的核酸扩增试验（NAAT）。此外，也有用于现场筛查的快速即时检测。
10. Treatment: Antiretroviral Therapy | 治疗：抗逆转录病毒治疗
event progression to AIDS. ART typically involves a combination of at least three drugs from different classes to target multiple stages of the HIV life cycle. Classes include reverse transcriptase inhibitors (nucleoside/nucleotide analogues such as zidovudine and tenofovir, and non‑ therapy (HAART) reduces the viral load to undetectable levels, allowing the immune system to recover. Adherence to the daily regimen is crucial; treatment interruptions can lead to drug resistance. When the viral load is undetectable, the risk of sexual transmission is effectively zero (U=U: Undetectable = Untransmittable).
目前尚无能够治愈HIV的方法，但抗逆转录病毒治疗（ART）可以有效抑制病毒复制、恢复CD4⁺细胞计数并防止其进展为艾滋病。ART通常联合使用至少三种来自不同类别的药物，以针对HIV生命周期的多个阶段。药物类别包括逆转录酶抑制剂（核苷/核苷酸类似物如齐多夫定和替诺福韦，以及非核苷类抑制剂）、整合酶链转移抑制剂、蛋白酶抑制剂和进入抑制剂。这种高效抗逆转录病毒治疗（HAART）可将病毒载量降至检测不到的水平，使免疫系统得以恢复。严格依从每日用药方案至关重要；中断治疗可能导致耐药。当病毒载量检测不到时，性传播风险实际上为零（U=U：测不到＝不传染）。
11. Prevention and Control Strategies | 预防与控制策略
Comprehensive prevention strategies are essential to control the spread of HIV. These include promoting safer sexual practices through condom use, implementing needle‑exchange programmes for injecting drug users, ensuring the safety of blood supplies by screening all donated blood for HIV antibodies or RNA, and providing voluntary male circumcision, which has been shown to reduce the risk of heterosexual acquisition. For HIV‑positive pregnant women, ART significantly reduces the risk of mother‑to‑child transmission to below 1%. Pre‑exposure prophylaxis (PrEP), a daily pill containing antiretroviral drugs, is highly effective for individuals at high risk of HIV exposure. Public education remains a cornerstone in combating stigma and encouraging testing.
全面的预防策略对于控制HIV传播至关重要。这些策略包括提倡使用安全套进行安全性行为、为注射吸毒者实施针具交换项目、通过筛查所有献血中的HIV抗体或RNA来确保血液安全，以及提供自愿男性包皮环切术（已证明可降低异性获得HIV的风险）。对于HIV阳性孕妇，ART可将母婴传播风险显著降低至1%以下。暴露前预防（PrEP）是一种包含抗逆转录病毒药物的每日口服药片，对于处于HIV暴露高风险的个人非常有效。公众教育仍然是消除歧视和鼓励检测的基石。
12. Challenges in Vaccine Development | 疫苗研发的挑战
Despite decades of research, no effective vaccine against HIV has been developed. The primary obstacle is the extreme genetic variability of the virus, which arises from the error‑prone nature of reverse transcriptase. This high mutation rate generates numerous viral variants within a single host, making it difficult for the immune system to recognise all strains. Furthermore, the virus directly targets and disables the very immune cells necessary for vaccine‑induced protection. Another challenge is the lack of a suitable animal model that perfectly mimics human HIV infection, although studies in non‑human primates using SIV (simian immunodeficiency virus) have provided valuable insights. Current research focuses on broadly neutralising antibodies and novel vaccine platforms, but a practical vaccine remains elusive.
尽管经过数十年的研究，至今尚未开发出有效的HIV疫苗。主要障碍是该病毒极高的遗传变异性，这源于逆转录酶易出错的性质。这种高突变率在单一宿主体内产生大量病毒变种，使免疫系统难以识别所有毒株。此外，病毒直接攻击并破坏的正是疫苗诱导保护所必需的免疫细胞。另一个挑战是缺乏能完美模拟人HIV感染的合适动物模型，尽管使用猴免疫缺陷病毒（SIV）在非人灵长类动物中进行的研究提供了宝贵的见解。当前的研究重点在于广谱中和抗体和新型疫苗平台，但实用的疫苗仍遥不可及。
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