📚 Year 12 Cambridge Biology: Core Concepts Overview | 剑桥AS生物：核心知识点梳理
The Cambridge International AS Level Biology syllabus lays the foundation for understanding life at the molecular, cellular, and systemic levels. In Year 12, students explore the structures and functions that sustain living organisms, from the intricate biochemical reactions inside cells to the coordinated physiology of whole plants and mammals. This article distils the essential topics, providing clear explanations, comparisons, and exam-focused insights to support your revision. Each section pairs an English explanation with its Chinese counterpart, ensuring you grasp the concepts in both languages.
剑桥国际AS阶段生物课程为从分子、细胞到系统层面理解生命奠定基础。Year 12的学生将探索维持生命体的结构与功能，从细胞内精妙的生化反应到整个植物与哺乳动物的协调生理活动。本文精炼核心主题，提供清晰的解释、对比以及面向考试的洞见，助力你的复习。每个部分均以中英双语对照呈现，确保你扎实掌握每个概念。
1. Cell Structure | 细胞结构
All living organisms are composed of cells, which can be broadly classified as prokaryotic or eukaryotic. Prokaryotic cells, such as bacteria, lack a membrane-bound nucleus and organelles; instead, they have a circular DNA molecule free in the cytoplasm, 70S ribosomes, and a cell wall made of peptidoglycan. Eukaryotic cells, found in animals, plants, and fungi, possess a true nucleus enclosed by a nuclear envelope, linear DNA associated with histones, 80S ribosomes, and numerous membrane-bound organelles such as mitochondria, the endoplasmic reticulum, and the Golgi apparatus.
所有生物都由细胞组成，细胞可大致分为原核细胞和真核细胞。原核细胞（如细菌）缺乏有膜包裹的细胞核与细胞器；它们具有游离于细胞质中的环状DNA分子、70S核糖体以及由肽聚糖构成的细胞壁。动物、植物和真菌的真核细胞则拥有由核膜包裹的真正细胞核、与组蛋白结合的线性DNA、80S核糖体以及多种具膜细胞器，例如线粒体、内质网和高尔基体。
Plant cells are distinguished by several unique features: a cellulose cell wall provides structural support; large permanent vacuoles maintain turgor pressure and store solutes; chloroplasts carry out photosynthesis using the green pigment chlorophyll. Animal cells lack these structures but have centrioles, which assist in spindle fibre formation during cell division.
植物细胞有几个独特特征：纤维素细胞壁提供结构支撑；巨大的中央液泡维持膨压并储存溶质；叶绿体利用绿色色素叶绿素进行光合作用。动物细胞没有这些结构，但有中心粒，在细胞分裂时协助纺锤体纤维的形成。
Viruses are acellular, non-living particles composed of a nucleic acid core (DNA or RNA) surrounded by a protein coat called a capsid. They lack ribosomes and enzymes for metabolism, so they must invade host cells and hijack their machinery to replicate. The tobacco mosaic virus (TMV) and human immunodeficiency virus (HIV) are typical examples studied in AS Biology.
病毒是非细胞结构的非生命颗粒，由核酸核心（DNA或RNA）和称为衣壳的蛋白质外壳组成。它们缺乏核糖体和代谢所需的酶，因此必须侵入宿主细胞并劫持其复制机制来繁殖。烟草花叶病毒（TMV）和人类免疫缺陷病毒（HIV）是AS生物中学习的典型例子。
| Feature | 特征 | Prokaryote | 原核生物 | Eukaryote | 真核生物 |
|---|---|---|
| Nucleus | 细胞核 | Absent | 无 | Present | 有 |
| DNA | DNA | Circular, naked | 环状、裸露 | Linear, histone-bound | 线性、与组蛋白结合 |
| Ribosomes | 核糖体 | 70S | 80S |
| Membrane-bound organelles | 具膜细胞器 | Absent | 无 | Present | 有 |
| Cell wall material | 细胞壁成分 | Peptidoglycan | 肽聚糖 | Cellulose (plants) / chitin (fungi) | 纤维素（植物）/ 几丁质（真菌） |
2. Biological Molecules | 生物大分子
Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in the ratio CnH2nOn. Monosaccharides such as glucose and fructose are simple sugars that serve as energy sources and building blocks. Two monosaccharides can join via a glycosidic bond in a condensation reaction to form disaccharides like maltose, sucrose, and lactose. Polysaccharides, including starch, glycogen, and cellulose, are polymers of glucose. Starch (a mixture of amylose and amylopectin) and glycogen are energy storage molecules; cellulose provides structural strength in plant cell walls because its β-glucose units form long, straight chains that hydrogen-bond into microfibrils.
碳水化合物是由碳、氢、氧组成的有机分子，通常符合CnH2nOn的比例。单糖如葡萄糖和果糖是简单的糖，可作为能量来源和构建单元。两个单糖可通过缩合反应形成糖苷键，连接成二糖，如麦芽糖、蔗糖和乳糖。多糖，包括淀粉、糖原和纤维素，是葡萄糖的聚合物。淀粉（直链淀粉和支链淀粉的混合物）和糖原是能量储存分子；纤维素为植物细胞壁提供结构强度，因为其β-葡萄糖单元形成长而直的链，通过氢键聚集成微纤丝。
Lipids are diverse hydrophobic molecules. Triglycerides, formed by ester bonds between glycerol and three fatty acids, are efficient energy stores. Phospholipids are similar but have one fatty acid replaced by a phosphate group, making them amphipathic; they form the basis of cell membranes. Cholesterol is a sterol that regulates membrane fluidity. The emulsion test identifies lipids: a cloudy white suspension appears when a sample is mixed with ethanol and then water.
脂质是多样的疏水性分子。甘油三酯由甘油和三个脂肪酸通过酯键连接而成，是高效的能量储存形式。磷脂与之类似，但一个脂肪酸被磷酸基团取代，使其具有两亲性；它们构成细胞膜的基础。胆固醇是一种调节膜流动性的甾醇。乳剂测试可鉴定脂质：将样品与乙醇混合再加水，产生乳白色悬浊液即为阳性。
Proteins are polymers of amino acids joined by peptide bonds. Each amino acid has a central carbon attached to an amino group, a carboxyl group, a hydrogen atom, and a variable R group. The primary structure is the sequence of amino acids; secondary structure refers to α-helices and β-pleated sheets stabilised by hydrogen bonds; tertiary structure is the overall 3D folding driven by hydrophobic interactions, ionic bonds, and disulfide bridges; quaternary structure involves the assembly of multiple polypeptide subunits, as seen in haemoglobin.
蛋白质是由肽键连接的氨基酸聚合物。每个氨基酸都有一个中心碳原子，连有氨基、羧基、氢原子和可变的R基团。一级结构是氨基酸的序列；二级结构是由氢键稳定的α-螺旋和β-折叠；三级结构是由疏水作用、离子键和二硫键驱动的整体三维折叠；四级结构涉及多个多肽亚基的组装，如血红蛋白。
Water’s properties—high specific heat capacity, latent heat of vaporisation, cohesion, and its action as a universal solvent—arise from hydrogen bonding between its polar molecules. These properties are vital for thermoregulation, transport, and as a medium for metabolic reactions.
水的性质——高比热容、高汽化潜热、内聚力以及作为万能溶剂的作用——源于其极性分子间的氢键。这些特性对温度调节、运输以及作为代谢反应的介质至关重要。
3. Enzymes | 酶
Enzymes are globular proteins that act as biological catalysts, lowering the activation energy of reactions without being consumed. Each enzyme has an active site with a specific shape complementary to its substrate. The induced-fit model states that the active site is not a rigid structure; upon substrate binding, it changes shape to strain bonds and orientate the substrate for catalysis. The Michaelis-Menten equation describes the relationship between substrate concentration and reaction rate:
酶是球状蛋白质，作为生物催化剂，降低反应的活化能而自身不被消耗。每种酶都有一个与底物形状互补的活性位点。诱导契合模型指出，活性位点并非刚性结构；底物结合时，它改变形状以拉扯化学键并使底物定向，从而促进催化。米氏方程描述了底物浓度与反应速率的关系：
V₀ = (Vmax [S]) / (Km + [S])
Temperature and pH affect enzyme activity. Initially, increasing temperature raises kinetic energy, so more frequent collisions lead to a higher rate. However, beyond the optimum temperature, the hydrogen and ionic bonds maintaining the tertiary structure break; the enzyme denatures irreversibly, and the active site is lost. Similarly, pH changes alter the charges on the amino acid R groups, disrupting ionic bonds and leading to denaturation. Each enzyme has an optimum pH—for example, pepsin in the stomach works best at pH 2.
温度和pH影响酶活性。起初，升高温度增加动能，更频繁的碰撞使反应速率提高。但超过最适温度后，维持三级结构的氢键和离子键断裂；酶不可逆地变性，活性位点丧失。同样，pH变化改变氨基酸R基团上的电荷，破坏离子键并导致变性。每种酶都有最适pH，例如胃中的胃蛋白酶在pH 2时活性最佳。
Competitive inhibitors have a shape similar to the substrate and bind to the active site, reducing the rate of reaction; this effect can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to a site other than the active site (allosteric site), altering the enzyme’s shape so the substrate cannot bind effectively; increasing substrate concentration does not overcome this inhibition. The rate approaches a lower Vmax.
竞争性抑制剂形状与底物相似，结合到活性位点，降低反应速率；这种抑制可通过增加底物浓度来克服。非竞争性抑制剂结合到活性位点以外的别构部位，改变酶的形状，使底物无法有效结合；增加底物浓度不能克服这种抑制，反应速率趋近于一个较低的Vmax。
4. Cell Membranes and Transport | 细胞膜与运输
The fluid mosaic model describes the cell membrane as a phospholipid bilayer with proteins embedded or attached. Phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails, forming a dynamic, flexible barrier. Proteins serve as channels, carriers, receptors, and enzymes. Cholesterol molecules fit between phospholipids, reducing fluidity at high temperatures and preventing solidification at low temperatures.
流动镶嵌模型将细胞膜描述为磷脂双分子层，其中镶嵌或附着有蛋白质。磷脂具有亲水的磷酸头部和疏水的脂肪酸尾部，形成动态、柔性的屏障。蛋白质充当通道、载体、受体和酶。胆固醇分子嵌在磷脂之间，降低高温时的流动性并防止低温下凝固。
Simple diffusion is the passive movement of small, non-polar molecules (e.g., O₂, CO₂) down their concentration gradient through the bilayer. Facilitated diffusion uses channel proteins (e.g., aquaporins) or carrier proteins to transport larger or charged molecules like glucose and ions, also down the gradient without energy input. Osmosis is the net movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Water potential (Ψ) is measured in kPa; pure water has Ψ = 0, and solutions have negative values.
简单扩散是小分子非极性物质（如O₂、CO₂）顺浓度梯度穿过双分子层的被动运输。易化扩散利用通道蛋白（如水孔蛋白）或载体蛋白运输较大或带电的分子，如葡萄糖和离子，同样顺梯度、不消耗能量。渗透是水通过部分透性膜从较高水势区域向较低水势区域的净移动。水势（Ψ）以千帕（kPa）为单位；纯水的水势为0，溶液的水势为负值。
Active transport moves molecules against their concentration gradient using energy from ATP. The sodium-potassium pump (Na⁺/K⁺-ATPase) is a classic example: it exports three Na⁺ ions out of the cell and imports two K⁺ ions in, per molecule of ATP hydrolysed. Bulk transport occurs via endocytosis (engulfing material into a vesicle) and exocytosis (fusing vesicles with the membrane to release contents).
主动运输利用ATP能量，将分子逆浓度梯度转运。钠钾泵（Na⁺/K⁺-ATP酶）是典型例子：每水解一个ATP分子，泵出三个Na⁺离子并泵入两个K⁺离子。大块物质通过胞吞作用（物质包裹入囊泡）和胞吐作用（囊泡与膜融合释放内容物）进行运输。
5. The Cell Cycle and Mitosis | 细胞周期与有丝分裂
The cell cycle consists of interphase and the mitotic phase. Interphase is divided into G₁ (cell growth, protein synthesis), S (DNA replication), and G₂ (organelle duplication, preparation for division). During mitosis, the nucleus divides into two genetically identical daughter nuclei. It is followed by cytokinesis, the division of the cytoplasm.
细胞周期包括间期和有丝分裂期。间期分为G₁期（细胞生长、蛋白质合成）、S期（DNA复制）和G₂期（细胞器复制、为分裂做准备）。有丝分裂过程中，细胞核分裂为两个遗传上相同的子细胞核。随后是胞质分裂，即细胞质的分裂。
Mitosis occurs in four stages: prophase (chromosomes condense, spindle fibres form, nuclear envelope breaks down), metaphase (chromosomes align at the equator, attached by centromeres to spindle fibres), anaphase (sister chromatids are pulled to opposite poles as spindle fibres shorten), and telophase (nuclear envelopes re-form, chromosomes decondense). In animal cells, a cleavage furrow accomplishes cytokinesis; in plant cells, a cell plate forms from vesicles derived from the Golgi apparatus.
有丝分裂分为四个阶段：前期（染色体凝集，纺锤体形成，核膜崩解），中期（染色体排列在赤道板上，着丝粒与纺锤丝相连），后期（纺锤丝缩短，姐妹染色单体被拉向两极），末期（核膜重新形成，染色体解凝集）。动物细胞通过分裂沟完成胞质分裂；植物细胞则由高尔基体来源的囊泡形成细胞板。
Uncontrolled mitosis can lead to cancer. Mutations in proto-oncogenes (which promote cell division) or tumour suppressor genes (which inhibit it) cause cells to divide uncontrollably, forming a tumour. Malignant tumours spread to other tissues via metastasis, while benign tumours remain localised.
有丝分裂失控可导致癌症。原癌基因（促进细胞分裂）或抑癌基因（抑制分裂）的突变使细胞不受控制地分裂，形成肿瘤。恶性肿瘤通过转移扩散到其他组织，而良性肿瘤仍保持局部化。
6. Nucleic Acids and Protein Synthesis | 核酸与蛋白质合成
DNA (deoxyribonucleic acid) stores genetic information. Its double helix consists of two antiparallel strands of nucleotides, each containing deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine). Complementary base pairing (A with T, C with G) via hydrogen bonds ensures accurate replication. RNA (ribonucleic acid) is single-stranded, contains ribose sugar, and uses uracil instead of thymine. Messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA) each play roles in protein synthesis.
DNA（脱氧核糖核酸）储存遗传信息。其双螺旋结构由两条反向平行的核苷酸链组成，每条核苷酸包含脱氧核糖、磷酸基团和含氮碱基（腺嘌呤、胸腺嘧啶、胞嘧啶或鸟嘌呤）。互补碱基配对（A与T，C与G）通过氢键确保复制准确性。RNA（核糖核酸）为单链，含核糖，用尿嘧啶代替胸腺嘧啶。信使RNA（mRNA）、转运RNA（tRNA）和核糖体RNA（rRNA）在蛋白质合成中各有作用。
Semi-conservative DNA replication occurs during the S phase of interphase. The enzyme helicase unwinds the double helix, and DNA polymerase adds complementary nucleotides to each exposed template strand in the 5′ to 3′ direction. The leading strand is synthesised continuously, while the lagging strand is synthesised in short Okazaki fragments, later joined by DNA ligase. Meselson and Stahl’s experiment using N isotopes confirmed the semi-conservative mechanism.
半保留DNA复制发生在间期的S期。解旋酶解开双螺旋，DNA聚合酶按5’到3’方向在每条暴露的模板链上添加互补核苷酸。前导链连续合成，滞后链则以短的冈崎片段合成，随后由DNA连接酶连接。Meselson和Stahl利用氮同位素的实验确认了半保留机制。
Transcription produces mRNA from a DNA template. RNA polymerase binds to a promoter region, unwinds the DNA, and synthesises a complementary mRNA strand using uracil in place of thymine. In eukaryotes, pre-mRNA is spliced to remove introns and join exons before leaving the nucleus. Translation occurs on ribosomes: the mRNA sequence is read in triplets (codons); tRNA molecules with complementary anticodons bring specific amino acids, which are joined by peptide bonds to form a polypeptide chain. The process continues until a stop codon is reached.
转录以DNA为模板生成mRNA。RNA聚合酶结合到启动子区域，解开DNA，利用尿嘧啶替代胸腺嘧啶合成互补的mRNA链。真核生物中，前体mRNA在离开细胞核前需剪接除去内含子并连接外显子。翻译在核糖体上进行：mRNA序列以三联体（密码子）阅读；带有互补反密码子的tRNA分子将特定氨基酸运来，通过肽键连接形成多肽链。该过程持续至遇到终止密码子。
7. Transport in Plants | 植物运输
Xylem tissue transports water and dissolved mineral ions from the roots to the leaves. Its vessels are made of dead, hollow cells with lignified walls, providing strength. The cohesion-tension theory explains water ascent: transpiration from leaves creates negative pressure (tension), and the cohesive property of water (due to hydrogen bonds) pulls the water column upward. Adhesion to xylem walls also assists. Transpiration is the evaporation of water from mesophyll cell surfaces into the air spaces, diffusing out through stomata.
木质部组织将水和溶解的矿质离子从根部运输到叶片。其导管由死亡、中空的细胞构成，细胞壁木质化，提供强度。内聚力-张力学说解释水上升的原因：叶片蒸腾产生负压（张力），水的内聚性（因氢键）将水柱向上拉。水与木质部壁的附着力也起辅助作用。蒸腾作用是水分从叶肉细胞表面蒸发到气室中，再通过气孔扩散出去的过程。
Phloem tissue transports sucrose and amino acids (assimilates) from sources (e.g., leaves) to sinks (e.g., roots, fruits). This process, called translocation, is explained by the mass flow hypothesis. At the source, companion cells actively load sucrose into sieve-tube elements, lowering water potential; water enters by osmosis, generating high hydrostatic pressure. At the sink, sucrose is removed, water follows, and pressure drops. The pressure gradient drives the flow of phloem sap.
韧皮部组织将蔗糖和氨基酸（同化物）从源（如叶片）运输到库（如根、果实）。该过程称为韧皮部转运，由压力流动假说解释。在源端，伴胞主动将蔗糖装载入筛管分子，降低水势；水分通过渗透进入，产生高静水压力。在库端，蔗糖被移走，水分随之离开，压力下降。压力梯度驱动韧皮部汁液流动。
Environmental factors such as light intensity, temperature, humidity, and wind speed influence the rate of transpiration. A potometer can measure water uptake, an indirect measure of transpiration rate. Xerophytes, plants adapted to dry conditions, show modifications like sunken stomata, rolled leaves, thick cuticles, and hairy leaves to reduce water loss.
光照强度、温度、湿度和风速等环境因素影响蒸腾速率。蒸腾计可测量吸水情况，从而间接测量蒸腾速率。旱生植物适应干燥环境，表现出气孔下陷、叶片卷曲、角质层厚和多毛叶片等特征以减少水分流失。
8. Transport in Mammals | 哺乳动物体内运输
Mammals have a closed, double circulatory system: the pulmonary circulation carries deoxygenated blood from the heart to the lungs and oxygenated blood back to the heart; the systemic circulation delivers oxygenated blood to the body and returns deoxygenated blood. The heart is a muscular organ with four chambers—right atrium, right ventricle, left atrium, and left ventricle—and valves (atrioventricular and semilunar) that prevent backflow.
哺乳动物具有封闭式双循环系统：肺循环将缺氧血从心脏送至肺部，再将富氧血送回心脏；体循环将富氧血输送到全身并收回缺氧血。心脏是一个由肌肉构成的四腔器官——右心房、右心室、左心房和左心室——以及防止回流的瓣膜（房室瓣和半月瓣）。
The cardiac cycle consists of atrial systole (atria contract, ventricles fill), ventricular systole (ventricles contract, atrioventricular valves close, semilunar valves open, blood ejected), and diastole (all chambers relax, semilunar valves close, atria refill). The sinoatrial node (SAN) in the right atrium acts as the pacemaker, initiating electrical impulses that cause atrial contraction; the atrioventricular node (AVN) delays the impulse to allow complete ventricular filling before ventricular systole.
心动周期包括心房收缩期（心房收缩、心室充盈）、心室收缩期（心室收缩，房室瓣关闭，半月瓣打开，血液射出）和舒张期（所有腔室松弛，半月瓣关闭，心房重新充盈）。右心房中的窦房结（SAN）充当起搏器，发出引发心房收缩的电脉冲；房室结（AVN）延迟脉冲，确保心室在收缩前完全充盈。
The three main types of blood vessel are arteries (thick muscular and elastic walls to withstand high pressure; carry blood away from the heart), veins (wider lumen, thinner walls, valves to prevent backflow; carry blood towards the heart), and capillaries (one-cell-thick walls for efficient exchange of substances). Blood consists of plasma (transporting nutrients, hormones, waste), red blood cells (biconcave discs containing haemoglobin for oxygen transport), white blood cells (immune defence), and platelets (blood clotting).
三种主要血管类型是动脉（管壁厚实、富有肌肉和弹性纤维以承受高压；将血液带离心脏）、静脉（管腔较宽、管壁较薄、有瓣膜防止回流；将血液带回心脏）和毛细血管（单细胞厚的管壁有利于高效的物质交换）。血液由血浆（运输营养物质、激素、废物）、红细胞（含血红蛋白的双凹圆盘形细胞，负责氧气运输）、白细胞（免疫防御）和血小板（参与凝血）组成。
9. Gas Exchange | 气体交换
The human gas exchange system includes the trachea, bronchi, bronchioles, and alveoli. The trachea and bronchi are supported by C-shaped cartilage rings, preventing collapse. Ciliated epithelial cells and goblet cells (secreting mucus) line the airways, trapping particles and pathogens and moving them upward to be swallowed, a mechanism called the mucociliary escalator.
人体气体交换系统包括气管、支气管、细支气管和肺泡。气管和支气管由C形软骨环支撑，防止塌陷。纤毛上皮细胞和杯状细胞（分泌黏液）衬在气道内壁，捕捉颗粒和病原体，并将其向上推送至咽部吞咽，这称为黏膜纤毛清除机制。
Alveoli are the sites of gas exchange. They are adapted by having a large surface area, a thin exchange surface (one cell of squamous epithelium and an endothelial cell), a steep concentration gradient maintained by ventilation and blood flow, and a rich capillary network. Oxygen diffuses into the blood and binds to haemoglobin to form oxyhaemoglobin (HbO₂); carbon dioxide diffuses from plasma into alveolar air. The coordination of breathing involves the diaphragm and intercostal muscles changing thoracic volume and pressure.
肺泡是气体交换的场所。其适应性特征包括巨大的表面积、极薄的交换表面（单层鳞状上皮和一层内皮细胞）、由通气和血流维持的陡峭浓度梯度以及丰富的毛细血管网。氧气扩散入血液并与血红蛋白结合形成氧合血红蛋白（HbO₂）；二氧化碳从血浆扩散至肺泡气。呼吸调节涉及膈肌和肋间肌改变胸腔容积与压力。
Tobacco smoke contains tar, nicotine, and carbon monoxide. Tar is carcinogenic and paralyses cilia, leading to inflammation and chronic obstructive pulmonary disease (COPD); nicotine is addictive and increases heart rate and blood pressure; carbon monoxide binds irreversibly to haemoglobin, reducing oxygen-carrying capacity. Long-term smoking increases the risk of lung cancer, emphysema (alveolar wall breakdown), and coronary heart disease.
烟草烟雾含有焦油、尼古丁和一氧化碳。焦油致癌且麻痹纤毛，导致炎症和慢性阻塞性肺病（COPD）；尼古丁具有成瘾性，增快心率、升高血压；一氧化碳不可逆地与血红蛋白结合，降低携氧能力。长期吸烟增加肺癌、肺气肿（肺泡壁破裂）和冠心病的风险。
10. Infectious Diseases and Immunity | 传染病与免疫
Pathogens include bacteria, viruses, fungi, and protoctists. Transmission can be direct (contact, droplet infection) or indirect (via vectors, contaminated water, or food). The body’s first line of defence is physical and chemical barriers: intact skin, mucus, stomach acid, and lysozyme in tears. When pathogens breach these, the non-specific inflammatory response is triggered: mast cells release histamine, causing vasodilation and increased capillary permeability, allowing phagocytes (neutrophils and macrophages) to reach the site. Phagocytes engulf pathogens by phagocytosis, fusing the phagosome with lysosomes where hydrolytic enzymes destroy the microbe.
病原体包括细菌、病毒、真菌和原生生物。传播方式可以是直接（接触、飞沫感染）或间接（通过媒介生物、受污染的水或食物）。人体的第一道防线是物理和化学屏障：完整的皮肤、黏液、胃酸以及泪液中的溶菌酶。当病原体突破这些屏障，非特异性炎症反应被触发：肥大细胞释放组胺，引起血管舒张和毛细血管通透性增加，使吞噬细胞（中性粒细胞和巨噬细胞）到达现场。吞噬细胞通过胞吞作用吞噬病原体，吞噬体与溶酶体融合，其中的水解酶摧毁微生物。
The specific immune response involves lymphocytes. B cells produce antibodies that bind to specific antigens on the pathogen’s surface, neutralising toxins, agglutinating pathogens for easier phagocytosis, and activating the complement system. T helper cells activate B cells and cytotoxic T cells; T killer cells destroy infected body cells. After an infection, memory cells remain, providing long-lasting immunity and a faster, stronger secondary response upon re-exposure.
特异性免疫反应涉及淋巴细胞。B细胞产生抗体，特异性结合病原体表面的抗原，中和毒素、凝集病原体便于吞噬，并激活补体系统。辅助性T细胞激活B细胞和细胞毒性T细胞；杀伤性T细胞摧毁受感染的体细胞。感染过后，记忆细胞存留，提供长效免疫，再次接触时引发更快更强的二次应答。
Vaccination introduces a harmless form of the antigen (attenuated pathogen, killed pathogen, or subunits) to stimulate the production of memory cells without causing disease. This confers artificial active immunity. Herd immunity occurs when a sufficient proportion of the population is immune, reducing pathogen spread. Antibiotics, such as penicillin, kill bacteria by inhibiting cell wall synthesis, but they are ineffective against viruses. Misuse of antibiotics has led to antibiotic resistance via natural selection.
疫苗接种引入无害形式的抗原（减毒病原体、灭活病原体或亚单位）以刺激记忆细胞产生，而不引发疾病，由此赋予人工主动免疫。当人群中有足够比例免疫时，便形成群体免疫，减少病原体传播。抗生素（如青霉素）通过抑制细胞壁合成杀死细菌，但对病毒无效。抗生素的滥用通过自然选择导致了抗生素耐药性。
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