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1 Overview of the Immune System
2 Innate Immunity
Q: What does H2 Biology Immunology (9477) cover? A: Innate and adaptive immunity, B and T lymphocyte responses, antibody structure, clonal selection, primary vs secondary immune response, vaccination, herd immunity, active vs passive immunity, ABO blood groups, allergies, and autoimmune diseases - all mapped to the SEAB 9477 Extension Topic A syllabus.
TL;DR Innate immunity responds quickly and non-specifically; adaptive immunity responds specifically and leaves memory. Exam answers usually need the sequence: antigen entry, antigen presentation, lymphocyte activation, effector response, and memory formation.
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What to take away
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Innate is fast; adaptive is specific.
10 seconds
Helper T cells coordinate many adaptive responses, so do not skip them in B cell or cytotoxic T cell answers.
100 seconds
Track antigen, APC, MHC, lymphocyte selection, effector cells, antibodies, and memory cells in order.
Concrete example: In vaccination, antigen exposure activates matching B and T cells, forms memory cells, and makes the secondary response faster and stronger after later exposure.
Immunology is a central component of Extension Topic A (Infectious Diseases) in the H2 Biology syllabus. It integrates cell biology, genetics, and molecular biology into one of the most exam-relevant themes. Questions on immune responses appear regularly in Papers 2, 3, and 4, and they reward students who can describe mechanisms precisely and link them to real-world applications such as vaccination and blood transfusion.
Syllabus scope: SEAB's current H2 Biology (9477) syllabus PDF is labelled for 2026. Extension Topic A (Infectious Diseases) covers innate immunity (physical/chemical barriers, phagocytosis, inflammation), specific immune response (humoral and cell-mediated), clonal selection and expansion, primary and secondary immune response, active and passive immunity, vaccination, herd immunity, blood groups and transfusion (ABO), allergies, and autoimmune diseases. [1]
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What this topic tests: Innate vs adaptive immunity, antigen presentation, clonal selection, B and T cell roles, antibody structure and effector functions, diversity mechanisms, vaccination, herd immunity threshold, ABO blood groups, autoimmunity, and allergies.
Top marks are lost by: Attributing memory to innate immunity; confusing antibody-mediated and cell-mediated responses; describing natural selection language when explaining clonal selection; omitting helper T cells from the B cell activation sequence; writing "antibodies kill pathogens" instead of describing opsonisation, neutralisation, or complement activation.
20-minute sprint plan: 5 min - flowchart of innate to adaptive response pathway; 5 min - antibody structure and effector functions; 5 min - primary vs secondary response graph; 5 min - vaccination principles and herd immunity threshold formula.
1 Overview of the Immune System
The immune system defends the body against pathogens through two main arms:
Innate immunity: Non-specific, rapid, and does not improve with repeated exposure.
Adaptive immunity: Specific to particular antigens, slower to activate on first exposure, and generates immunological memory.
Both arms work together. Innate responses contain infection early and activate adaptive responses through antigen presentation. The timeline of response is a common exam focus: innate responses begin within minutes to hours; adaptive responses take days to weeks on first exposure but are dramatically faster on re-exposure due to memory cells.
Mucous membranes: Trap pathogens in the respiratory, gastrointestinal, and urogenital tracts. Cilia in the respiratory tract sweep mucus upward (mucociliary escalator).
Chemical defences: Lysozyme in tears and saliva degrades bacterial cell walls; stomach acid (low pH) destroys ingested microbes.
Microbiota: Commensal bacteria compete with pathogens for nutrients and attachment sites.
2.2 Cellular components
Phagocytes (neutrophils, macrophages): Engulf and digest pathogens through phagocytosis. Macrophages also function as antigen-presenting cells (APCs), linking innate and adaptive immunity.
Natural killer (NK) cells: Recognise and destroy virus-infected cells and some tumour cells by inducing apoptosis. NK cells detect the loss of MHC class I expression (a common viral strategy to avoid detection by cytotoxic T cells).
Dendritic cells: Highly efficient APCs that migrate to lymph nodes to activate naive T cells, forming the bridge between innate and adaptive immunity.
2.3 Pattern recognition
Innate immune cells detect conserved microbial structures called pathogen-associated molecular patterns (PAMPs) using pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs). This recognition is broad rather than antigen-specific.
2.4 Inflammatory response
Tissue damage and infection trigger inflammation. The sequence of events is a frequent structured-question target:
Damaged cells and mast cells release histamine and cytokines (signalling molecules).
Blood vessels dilate (vasodilation causes redness and heat) and become more permeable (fluid leaks into tissue causing swelling).
Phagocytes are attracted to the site of infection by chemokines (chemotaxis).
Phagocytes engulf and destroy pathogens and debris.
The inflammatory response contains the infection and initiates tissue repair.
The four cardinal signs of inflammation are: redness, heat, swelling, and pain.
3 Adaptive Immunity
3.1 Key features
Feature
Description
Specificity
Each lymphocyte recognises a single antigen via unique surface receptors
Diversity
The combined repertoire can recognise an enormous number of antigens
Memory
Memory cells enable faster and stronger secondary responses
Self-tolerance
Lymphocytes attacking self-antigens are deleted during maturation (clonal deletion)
3.2 Antigens and antigen presentation
An antigen is any molecule (typically a protein or polysaccharide on the pathogen surface) that can be recognised by the adaptive immune system and trigger an immune response. Antigen-presenting cells (APCs) process antigens and display fragments on their surface using major histocompatibility complex (MHC) molecules:
MHC class I: Found on all nucleated cells; presents intracellular antigens (e.g. from viruses replicating inside the cell) to cytotoxic T cells. This allows the immune system to detect infected "self" cells.
MHC class II: Found on APCs (dendritic cells, macrophages, B cells); presents extracellular antigens to helper T cells.
3.3 Clonal selection and expansion
When a lymphocyte encounters its matching antigen, it is selected (clonal selection) and undergoes rapid proliferation (clonal expansion), producing a large population of:
Effector cells - short-lived cells that carry out the immediate immune response (plasma cells, cytotoxic T cells).
Memory cells - long-lived cells that persist after the infection clears, enabling a faster secondary response.
Exam language note: Clonal selection selects from pre-existing lymphocyte clones present at birth. It is NOT analogous to Darwinian natural selection across generations. Do not say lymphocytes "evolve" or "mutate to resist the pathogen."
4 T Cell Responses (Cell-Mediated Immunity)
Cell-mediated immunity is critical for eliminating intracellular pathogens, virus-infected cells, and tumour cells.
4.1 Helper T cells (CD4+)
Recognise antigens presented on MHC class II by APCs.
Become activated and secrete cytokines.
Cytokines activate B cells (triggering antibody production), activate and support cytotoxic T cells, and stimulate macrophages.
Helper T cells are the central coordinators of the adaptive immune response. Without them, neither the humoral nor the cell-mediated arm functions effectively. This is why HIV (which destroys CD4+ T cells) leads to immunodeficiency.
4.2 Cytotoxic T cells (CD8+)
Recognise antigens presented on MHC class I by infected host cells.
Kill infected cells by releasing:
Perforin - polymerises to form pores in the target cell membrane, disrupting the osmotic gradient and triggering cell death.
Granzymes - serine proteases that enter through perforin pores and activate caspases, inducing apoptosis.
After killing, the cytotoxic T cell moves on to destroy additional infected cells.
4.3 Memory T cells
Long-lived cells that persist in lymphoid tissue after infection is cleared.
Enable a faster and more robust secondary response on re-exposure.
Include both memory helper T cells and memory cytotoxic T cells.
5 B Cell Responses (Antibody-Mediated / Humoral Immunity)
Humoral immunity targets extracellular pathogens, toxins, and free viral particles.
5.1 Activation and differentiation - the full sequence
B cell binds antigen via its B cell receptor (BCR, a membrane-bound antibody).
The B cell internalises the antigen-BCR complex, processes the antigen, and displays fragments on MHC class II.
A helper T cell that recognises the same antigen on MHC class II provides co-stimulatory signals and cytokines.
The activated B cell undergoes clonal expansion.
Clonal expansion produces plasma cells (effector B cells) and memory B cells.
5.2 Plasma cells
Short-lived effector cells (survive days to weeks).
Secrete large quantities of antibodies (up to thousands of molecules per second per cell).
Antibodies circulate in blood and lymph, targeting extracellular pathogens and toxins.
5.3 Memory B cells
Long-lived cells that persist in lymphoid tissue.
On re-exposure to the same antigen, differentiate rapidly into plasma cells.
Produce antibodies faster and at higher concentrations than during the primary response.
Affinity maturation means memory B cell antibodies typically bind the antigen more tightly than primary-response antibodies.
6 Antibody Structure
6.1 IgG architecture
The basic antibody (immunoglobulin G) is a Y-shaped glycoprotein consisting of:
Two heavy chains and two light chains, connected by disulfide bonds.
Variable (V) regions at the tips of the Y: form the antigen-binding sites (Fab regions). The variable region determines antibody specificity. Each arm of the Y is one Fab region.
Constant (C) regions: form the stem of the Y (Fc region). The Fc region mediates effector functions - complement activation and binding to phagocyte Fc receptors (opsonisation).
Hinge region: flexible segment between the Fab arms and Fc stem, allowing the two arms to bind antigens at varying distances apart.
6.2 Effector functions
Neutralisation: Antibodies bind to pathogens or toxins, sterically blocking their ability to attach to host cell receptors. This prevents infection or toxin action.
Opsonisation: Antibodies coat pathogens; phagocytes have receptors for the Fc region and bind opsonised targets more efficiently, enhancing phagocytosis.
Complement activation: Antibody-antigen complexes trigger the classical complement cascade, producing the membrane attack complex (MAC) that lyses pathogens, and complement fragments that further opsonise targets.
Agglutination: Multi-valent antibodies cross-link multiple pathogens, forming large clumps that are more easily phagocytosed.
7 Antibody Diversity
The immune system generates millions of distinct antibody specificities. Three main mechanisms are required knowledge for 9477: [1]
7.1 Somatic recombination (V(D)J recombination)
Immunoglobulin genes are organised in segments - V (variable), D (diversity), and J (joining). During B cell development in the bone marrow, one segment from each group is randomly selected and joined together to form the variable region of the antibody gene. The large number of possible combinations generates millions of distinct antigen-binding specificities before any antigen is encountered.
7.2 Somatic hypermutation and affinity maturation
After antigen exposure, the variable regions of active B cell immunoglobulin genes undergo point mutations at a high rate in the germinal centres of lymph nodes. B cells whose mutations increase binding affinity for the antigen are preferentially selected to survive and proliferate - this is affinity maturation. The result is progressively higher-affinity antibodies over the course of an immune response.
7.3 Class switching (isotype switching)
The constant region of the heavy chain (which determines the antibody class - IgM, IgG, IgA, IgE) can be changed without altering the variable region. The antigen specificity remains identical, but the effector function changes. For example, switching from IgM (produced first in a primary response) to IgG (dominant in secondary responses and most abundant in blood) is directed by cytokines from helper T cells.
8 Primary and Secondary Immune Responses
8.1 Primary response
On first exposure to an antigen:
There is a lag period of several days while naive lymphocytes are activated, clonally expanded, and differentiated.
IgM is produced first, followed by IgG (after class switching).
Antibody levels rise, plateau, and then decline as plasma cells die.
Memory B cells and memory T cells are generated.
8.2 Secondary response
On subsequent exposure to the same antigen:
The lag period is much shorter (memory cells respond rapidly and do not need to go through initial activation from scratch).
Antibody levels peak at much higher concentrations and are sustained for longer.
IgG predominates; affinity is typically higher due to previous affinity maturation in memory B cells.
This is the mechanistic basis for the effectiveness of vaccination and for why recovered individuals are protected from re-infection.
8.3 Graph analysis tip
A typical Paper 2 question provides an antibody titre graph with two peaks after two exposures. Key features to identify and explain:
Longer lag period and lower peak for primary response - explain in terms of naive lymphocyte activation time.
Shorter lag period and higher, sustained peak for secondary response - explain in terms of memory cells and their pre-activation state.
Predominance of IgG in secondary response vs IgM in primary - link to class switching and affinity maturation.
9 Vaccination
9.1 Principles
A vaccine introduces antigens (weakened, inactivated, or subunit forms of a pathogen, or genetic instructions to produce a specific antigen) to stimulate adaptive immunity without causing disease. The immune system mounts a primary response and generates memory cells. Upon subsequent exposure to the actual pathogen, the secondary response provides rapid and effective protection before significant disease develops.
9.2 Types of vaccines
Type
Mechanism
Example
Notes
Live attenuated
Weakened but replicating pathogen
MMR (measles, mumps, rubella)
Strong, long-lasting immunity; may be contraindicated in immunocompromised patients
Inactivated
Killed pathogen
Flu (some formulations), polio (Salk)
Safer; often requires booster doses
Subunit/recombinant
Purified antigenic protein
Hepatitis B surface antigen
Highly targeted; no risk of infection
mRNA
Cells produce target antigen from instructions
COVID-19 vaccines
Rapid manufacturing; no live pathogen needed
Toxoid
Inactivated toxin
Tetanus, diphtheria
Trains immune system against toxin
9.3 Herd immunity
When a sufficient proportion of a population is immune, the pathogen cannot find enough susceptible hosts to maintain transmission chains, and spread declines even among unvaccinated individuals (the young, the elderly, and immunocompromised people who cannot receive certain vaccines).
The herd immunity threshold pc is related to the basic reproduction number R0 (the average number of secondary infections caused by one infected person in a fully susceptible population):
pc=1−R01
For measles (R0≈12−18), pc≈92−95. For COVID-19 (original strain, R0≈2−3), pc≈50−67. These numbers illustrate why high measles vaccination coverage is essential.
9.4 Limitations and challenges
Rapidly mutating pathogens (e.g. influenza with antigenic drift and antigenic shift) may require frequent vaccine reformulation.
Some pathogens actively suppress immune responses (e.g. HIV destroys CD4+ T cells, undermining vaccine-induced immunity).
Cold chain logistics can limit vaccine distribution in low-resource settings.
Vaccine hesitancy affects population coverage and herd immunity thresholds.
10 Active vs Passive Immunity
Feature
Active immunity
Passive immunity
How acquired
Exposure to antigen (infection or vaccination)
Transfer of pre-formed antibodies
Memory cells produced
Yes
No
Duration
Long-lasting (years to lifelong)
Short-term (weeks to months)
Time to protection
Days to weeks (primary) or hours (secondary)
Immediate
Natural example
Recovery from infection
Maternal IgG via placenta; IgA via breast milk
Artificial example
Vaccination
Injection of antibody preparations (e.g. anti-tetanus immunoglobulin, anti-venom)
11 ABO Blood Groups and Transfusion
ABO blood groups are determined by the presence or absence of specific glycoprotein antigens (agglutinogen A and agglutinogen B) on the surface of red blood cells, encoded by alleles at the ABO locus.
Blood group
Antigen on RBC
Antibody in plasma
Can donate to
Can receive from
A
A
Anti-B
A, AB
A, O
B
B
Anti-A
B, AB
B, O
AB
A and B
Neither
AB only
A, B, AB, O (universal recipient)
O
Neither
Anti-A and anti-B
A, B, AB, O (universal donor)
O only
Why incompatible transfusions cause reactions
Antibodies (agglutinins) present in the recipient's plasma bind to the mismatched antigens (agglutinogens) on the donated red blood cells. This triggers:
Agglutination - red blood cells clump together, blocking capillaries.
Complement activation - the antibody-antigen complex activates complement, lysing the donated red blood cells (haemolysis).
Release of haemoglobin into circulation - causes kidney damage and potentially fatal haemolytic transfusion reaction.
Exam note: The antibodies against ABO antigens are naturally occurring (they develop in infancy, apparently triggered by cross-reactive antigens from gut bacteria), not formed through prior exposure to blood. This is unlike most antibody responses, which require prior antigen exposure.
12 Allergies and Autoimmune Diseases
12.1 Allergies (hypersensitivity)
An allergy is an exaggerated immune response to a harmless antigen (allergen) such as pollen, dust mites, or peanut proteins.
Mechanism (IgE-mediated, type I hypersensitivity):
First exposure to the allergen - the immune system produces IgE antibodies specific to the allergen (sensitisation).
IgE antibodies bind to receptors on mast cells and basophils throughout the body.
On re-exposure, the allergen cross-links IgE molecules on mast cell surfaces.
Cross-linking triggers mast cell degranulation - release of histamine, leukotrienes, and other inflammatory mediators.
Symptoms range from mild (runny nose, itching) to severe and life-threatening (anaphylaxis).
Antihistamines block histamine receptors, reducing allergy symptoms. Adrenaline (epinephrine) is the emergency treatment for anaphylaxis - it reverses bronchospasm, vasodilation, and oedema.
12.2 Autoimmune diseases
Autoimmunity occurs when the immune system fails to maintain self-tolerance and mounts a destructive response against the body's own cells and tissues.
During lymphocyte maturation, cells that react strongly against self-antigens are normally deleted (clonal deletion). Autoimmune diseases arise when this process fails.
Examples relevant to the 9477 syllabus context:
Type 1 diabetes: Cytotoxic T cells destroy the insulin-producing beta cells of the pancreatic islets.
Rheumatoid arthritis: Antibodies and T cells attack joint tissues.
Treatment of autoimmune diseases often requires immunosuppressive drugs, which carry the tradeoff of reduced ability to fight infections.
13 Worked Examples and Essay Scaffolds
Worked Example 1 - 6-mark structured question: Describe how the body responds to a bacterial infection using innate and adaptive immunity.
Mark-scoring answer structure:
Bacteria breach first-line defences; damaged cells release histamine and cytokines, triggering inflammation (vasodilation, increased permeability, phagocyte recruitment). [2 marks - innate response with named mediators]
Phagocytes (neutrophils, macrophages) engulf bacteria by phagocytosis. Macrophages/dendritic cells process bacterial antigens and display them on MHC class II (antigen presentation). [1 mark - APC function]
Helper T cells (CD4+) recognise antigen on MHC class II, become activated, and secrete cytokines. [1 mark - T cell activation]
Cytokines activate B cells, which undergo clonal selection, clonal expansion, and differentiate into plasma cells and memory B cells. [1 mark - B cell activation and clonal selection]
Plasma cells secrete antibodies that neutralise, opsonise, or agglutinate bacteria. Memory cells persist for secondary responses. [1 mark - effector and memory outcome]
Common omissions: forgetting antigen presentation; skipping helper T cells; not mentioning memory cells; saying "antibodies kill bacteria" without explaining mechanism.
Worked Example 2 - 8-mark essay scaffold: "Explain how vaccination provides protection against infectious diseases."
Paragraph structure:
Paragraph 1 - Vaccine introduces antigen (2 marks):
State that vaccines deliver antigens (weakened/inactivated pathogen or specific protein) without causing disease. Define the types briefly (live attenuated, inactivated, subunit, mRNA). State that this triggers a primary immune response.
Paragraph 2 - Primary response and memory generation (3 marks):
Antigen presentation by APCs activates helper T cells, which activate B cells and cytotoxic T cells. Clonal selection and expansion occur. Plasma cells produce antibodies; memory B cells and memory T cells are generated and persist long-term.
Paragraph 3 - Secondary response on real infection (2 marks):
On exposure to the actual pathogen, memory cells enable a rapid secondary response. Antibody levels rise quickly (shorter lag period, higher concentration). The pathogen is neutralised before significant symptoms develop.
Worked Example 3 - 12-mark evaluative essay scaffold: "Evaluate the effectiveness of the immune system in protecting the body against disease, referring to both innate and adaptive immunity."
Introduction (1 mark): Define innate and adaptive immunity; state both are needed for complete protection; preview that the question asks for critical evaluation.
Innate immunity - strengths (2 marks): Rapid (minutes to hours); broad recognition via PRRs/TLRs; physical barriers provide constant protection; no prior exposure needed.
Innate immunity - limitations (1 mark): Non-specific; pathogens can evolve mechanisms to evade (e.g. Mycobacterium tuberculosis survives inside macrophages; viruses evade NK cells by MHC mimicry).
Adaptive immunity - limitations (2 marks): Slow primary response (days); pathogens with high mutation rates (influenza, HIV) can escape recognition; HIV specifically disables the adaptive arm by destroying CD4+ T cells.
Integration and higher-level evaluation (2 marks): The arms are interdependent - innate responses activate adaptive via APCs; adaptive responses enhance innate via cytokines and opsonisation. Failure at the interface (e.g. absent dendritic cell migration) impairs the adaptive response. Reference the primary-secondary response graph as evidence of adaptive immunity's effectiveness on second exposure.
Conclusion (1 mark): The immune system is highly effective overall but not infallible; pathogens are under continuous selective pressure to evade detection, and the arms are only as effective as the weakest link.
14 Common Exam Pitfalls
Attributing memory to innate immunity: Memory cells are a feature of adaptive immunity only. Innate responses are the same on every encounter.
Confusing cell-mediated and antibody-mediated immunity: Cytotoxic T cells kill infected host cells directly. Antibodies target extracellular pathogens and toxins. Neither can substitute for the other.
"Antibodies kill pathogens directly": Antibodies neutralise, opsonise, agglutinate, or activate complement. They do not directly lyse pathogens by themselves in most cases. Name the mechanism.
Using natural selection language for clonal selection: Clonal selection occurs within an individual's lifespan, selecting from pre-existing lymphocyte clones. It is not the same as natural selection acting on a species over generations.
Omitting helper T cells: Many students jump from antigen presentation straight to B cell antibody production. The helper T cell step (cytokine secretion) is required for full credit.
Vaccination = antibodies, not memory cells: Full-mark answers state that vaccines generate memory cells, enabling a secondary response. Simply saying "antibodies are produced" misses the mechanism.
Forgetting ABO antibodies are naturally occurring: Students sometimes write that anti-A or anti-B antibodies form after prior exposure to incompatible blood. They are present from infancy without any prior blood exposure.
Autoimmunity vs allergy confusion: Autoimmunity targets self-antigens. Allergies target harmless foreign antigens (allergens). Different mechanisms, different cells (autoimmunity: T cells, IgG; allergy: IgE, mast cells).
15 Cross-Topic Links
Cell biology (Core Idea 1): Phagocytosis, membrane receptors, cell signalling (cytokines), endocytosis (antigen processing), and apoptosis (perforin/granzyme pathway).
Genetics (Core Idea 2): Somatic recombination (V(D)J) involves programmed gene rearrangement; somatic hypermutation is a form of localised, targeted mutation; ABO blood group genetics (codominance, multiple alleles).
Evolution (Core Idea 4): Pathogens evolve under selective pressure from the immune system (antigenic variation in influenza, HIV). Antibiotic resistance is driven by natural selection in bacterial populations. Immunological arms races shape both host and pathogen evolution.
Extension Topic A - Infectious Diseases: Immunology is the mechanistic foundation for understanding pathogen biology, vaccination, epidemiology, and public health strategy.
16 How This Topic Appears in Papers 2, 3, and 4
Paper 2 (Structured questions): Antibody titre graphs with primary and secondary responses; ELISA data interpretation; comparison of immune responses with and without vaccination; sequence-of-events questions from antigen entry to effector response; blood group compatibility questions.
Paper 3 (Free-response essays): "Describe how the adaptive immune system responds to a bacterial infection," "Evaluate the role of vaccination in controlling infectious diseases," "Explain how the immune system distinguishes between self and non-self." Integration with genetics (diversity mechanisms) and evolution (pathogen adaptation) earns higher bands.
Paper 4 (Practical): ELISA simulations, serial dilutions of antibody solutions, data from immune cell assays. Focus on MMO/PDO/ACE evidence strands.
Quiz - Test Yourself
Quick Retrieval Check
Name three features that distinguish adaptive immunity from innate immunity.
Explain the full sequence of events from bacterial antigen entry to plasma cell antibody secretion. Include the role of helper T cells.
Describe two mechanisms that generate antibody diversity.
Using an antibody titre graph, explain why the secondary immune response provides faster protection than the primary response.
Why is herd immunity important for individuals who cannot be vaccinated?
A patient with blood group A receives a transfusion of group B blood. Describe the molecular events that would cause a transfusion reaction.
Explain how IgE antibodies are involved in an allergic reaction.
Give two reasons why HIV infection leads to susceptibility to opportunistic infections.
Need help mastering Immunology? Our H2 Biology tuition programme covers this topic with structured essay practice, DBQ technique drills, and Paper 4 practical preparation.
FAQ
How does the immune system work? The immune system uses two main arms. The innate immune system (phagocytes, NK cells, inflammatory response) provides rapid, non-specific defence that activates within minutes to hours. The adaptive immune system (B and T lymphocytes) mounts a specific response over days to weeks, targeting a particular antigen, and generates immunological memory so that future encounters with the same pathogen trigger a much faster and stronger response.
What is the difference between innate and adaptive immunity? Innate immunity is non-specific, rapid, and provides the same response on every encounter. It uses pattern recognition to detect broad classes of pathogens. Adaptive immunity is highly specific to individual antigens, takes longer on first exposure, but generates memory. On re-exposure, the adaptive response is dramatically faster due to memory B and T cells. The two arms are interdependent - innate responses activate adaptive immunity through antigen presentation.
What is clonal selection? Clonal selection is the process by which a lymphocyte (B or T cell) that carries a receptor matching a specific antigen is selected and stimulated to proliferate. This generates a large clone of cells all identical in their antigen specificity - some become effector cells that carry out the immediate response, and some become memory cells for future encounters. Clonal selection explains why adaptive immunity is both specific (only matching cells are expanded) and memorised (those cells persist).
How do vaccines work? Vaccines introduce antigens from a pathogen (in weakened, inactivated, subunit, or mRNA-encoded form) without causing disease. The immune system mounts a primary response: APCs present the antigen, helper T cells are activated, B cells undergo clonal selection and expansion, and plasma cells produce antibodies. Memory B cells and memory T cells are generated. On subsequent exposure to the real pathogen, these memory cells enable a rapid secondary response - high-affinity antibody production within hours - neutralising the pathogen before disease develops.
What is herd immunity and how is it calculated? Herd immunity is the indirect protection of unvaccinated individuals when a sufficiently large proportion of a population is immune, preventing pathogen transmission chains from being sustained. The herd immunity threshold pc is calculated as pc=1−1/R0, where R0 is the basic reproduction number of the pathogen. For measles with R0≈15, approximately 93% of the population must be immune to achieve herd immunity.
What is the difference between active and passive immunity? Active immunity results from the body's own immune response to an antigen (through infection or vaccination). It generates memory cells and is long-lasting (years to life). Passive immunity results from receiving pre-formed antibodies (e.g. maternal antibodies transferred via the placenta, or injected immunoglobulins for emergency treatment). It provides immediate protection but is short-lived (weeks to months) and generates no memory cells.
What is an autoimmune disease? An autoimmune disease occurs when the immune system fails to distinguish self from non-self and attacks the body's own tissues. Normally, lymphocytes that react to self-antigens are deleted during maturation (clonal deletion). When this process fails, autoreactive T cells or autoantibodies cause chronic inflammation and tissue destruction. Examples include type 1 diabetes (beta cells of pancreas), rheumatoid arthritis (joint tissue), and multiple sclerosis (myelin sheaths).
How do allergies develop? Allergies arise from an exaggerated IgE-mediated immune response to harmless antigens (allergens). On first exposure, the immune system produces IgE antibodies that bind to mast cells (sensitisation). On re-exposure, the allergen cross-links IgE molecules on mast cells, triggering degranulation and histamine release. This causes the symptoms of allergy - vasodilation, increased vascular permeability, mucus secretion, and smooth muscle contraction. In severe cases (anaphylaxis), systemic histamine release is life-threatening.
Where can I find the full H2 Biology Notes series? Start at the H2 Biology Notes hub, then follow Core Ideas 1-4 and the Extension Topics.
Is immunology in the core syllabus or an extension topic? Immunology falls under Extension Topic A (Infectious Diseases) in the SEAB 9477 syllabus. It integrates heavily with Core Ideas 1, 2, and 4. Extension topics together form approximately 15% of the total curriculum. [1]
Do I need to know about specific cytokines by name? The 9477 syllabus does not require memorisation of specific cytokine names. You should understand their general roles: helper T cells secrete cytokines that activate B cells, cytotoxic T cells, and macrophages. For highest-band essays, you can refer to "cytokines including interleukins" to show breadth without needing to memorise individual names.
