Non-Specific Immune Response

The non-specific (innate) immune response provides immediate, generalised defence against any pathogen that breaches the body's barriers. Unlike the specific immune response, it does not target particular pathogens and does not involve immune memory. This lesson covers the physical barriers, phagocytosis, inflammation, and other non-specific mechanisms for the Edexcel A-Level Biology (9BI0) specification.

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Overview of the Immune System

The immune system can be divided into two main categories:

Type	Features	Speed	Specificity	Memory
Non-specific (innate)	Same response to any pathogen; physical barriers, phagocytosis, inflammation	Immediate (minutes to hours)	None (acts against all pathogens)	No
Specific (adaptive)	Targets specific antigens; involves B cells and T cells	Slower (days to weeks on first exposure)	Highly specific	Yes (immune memory)

The non-specific immune system acts as the first and second lines of defence.

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First Line of Defence: Physical and Chemical Barriers

These barriers prevent pathogens from entering the body in the first place.

Barrier	Location	How it protects
Skin	External surface	Tough, keratinised epithelium provides a physical barrier; dry surface inhibits microbial growth
Mucous membranes	Respiratory, digestive, and urogenital tracts	Mucus traps pathogens and particles
Cilia	Lining the trachea and bronchi	Ciliated epithelial cells beat rhythmically to move mucus (and trapped pathogens) up towards the throat to be swallowed (mucociliary escalator)
Stomach acid (HCl, pH ~2)	Stomach	Denatures proteins and kills most ingested pathogens
Lysozyme	Tears, nasal secretions, saliva	An enzyme that hydrolyses the peptidoglycan in bacterial cell walls, causing lysis
Sebum	Secreted by sebaceous glands in the skin	Oily secretion that maintains a low pH on the skin surface, inhibiting bacterial growth
Commensal (normal) flora	Gut, skin, vagina	Compete with pathogens for nutrients and space; produce antimicrobial substances

Exam Tip: Lysozyme specifically targets the peptidoglycan (murein) in bacterial cell walls. This is why bacteria, not viruses, are susceptible to lysozyme. Always name the target molecule in your answer.

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Second Line of Defence: Non-Specific Cellular and Chemical Responses

If pathogens breach the first line of defence, the non-specific immune system mounts an internal response.

Phagocytosis

Phagocytosis is the process by which specialised white blood cells (phagocytes) engulf and destroy pathogens.

Types of Phagocytes

Cell type	Location	Key features
Neutrophils	Blood; first to arrive at infection site	Most abundant phagocyte; short-lived; form pus
Macrophages	Tissues (derived from blood monocytes)	Larger; longer-lived; also act as antigen-presenting cells (APCs)

Steps of Phagocytosis

1. Chemotaxis: The phagocyte is attracted to the site of infection by chemical signals (e.g. cytokines, complement proteins, bacterial products).
2. Recognition and attachment: The phagocyte recognises pathogen-associated molecular patterns (PAMPs) on the pathogen surface using pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs). Opsonisation (coating the pathogen with antibodies or complement proteins) enhances attachment.
3. Engulfment: The phagocyte extends pseudopodia (extensions of the cell membrane) around the pathogen.
4. Phagosome formation: The pathogen is enclosed in a phagosome (a membrane-bound vesicle inside the cell).
5. Lysosome fusion: Lysosomes fuse with the phagosome to form a phagolysosome.
6. Digestion: Lysosomal enzymes (hydrolases, proteases, lysozyme) and reactive oxygen species destroy and digest the pathogen.
7. Antigen presentation: The macrophage processes pathogen antigens and displays them on its surface bound to MHC class II proteins. This activates the specific immune response (T helper cells).

The following diagram summarises the stages of phagocytosis:

graph TD
    A["Pathogen detected"] --> B["Phagocyte attracted<br/>(chemotaxis)"]
    B --> C["Pathogen engulfed<br/>(endocytosis)"]
    C --> D["Phagosome formed"]
    D --> E["Lysosome fuses<br/>(enzymes digest pathogen)"]
    E --> F["Antigens presented<br/>on cell surface"]

Exam Tip: The final step — antigen presentation — is the critical link between the non-specific and specific immune responses. When describing phagocytosis, always include this step to show your understanding of how innate immunity triggers adaptive immunity.

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The Inflammatory Response

Inflammation is a localised, non-specific response to tissue damage or infection.

Signs of Inflammation

Sign	Cause
Redness	Increased blood flow (vasodilation) to the area
Heat	Increased blood flow; raised metabolic activity
Swelling (oedema)	Increased permeability of blood capillaries allows fluid and white blood cells to leak into tissues
Pain	Stimulation of pain receptors by chemical mediators (e.g. prostaglandins, bradykinin)

Mechanism

1. Damaged cells and mast cells release histamine and other chemical mediators.
2. Histamine causes vasodilation of local arterioles (increased blood flow → redness and heat).
3. Histamine also increases the permeability of capillary walls, allowing plasma proteins (including complement and antibodies) and phagocytes to move into the infected tissue.
4. Neutrophils are the first phagocytes to arrive (within minutes), followed by monocytes (which develop into macrophages in the tissues).
5. Cytokines (signalling proteins) are released by macrophages and other cells, further recruiting immune cells.

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Fever

• A systemic (whole-body) response to infection.
• Pyrogens (fever-inducing substances) — either from pathogens (exogenous) or from the body's own immune cells (endogenous, e.g. interleukin-1) — act on the hypothalamus to raise the body's thermostat.
• A moderate fever may be beneficial: it increases the rate of immune cell activity and may inhibit pathogen replication.

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The Complement System

The complement system is a group of over 30 plasma proteins (produced by the liver) that circulate in the blood in an inactive form. They are activated by the presence of pathogens and have several functions:

Function	Detail
Opsonisation	Complement proteins coat the surface of pathogens, making them easier for phagocytes to recognise and engulf
Chemotaxis	Complement fragments attract phagocytes to the site of infection
Membrane attack complex (MAC)	Complement proteins form pores in the pathogen's cell membrane, causing lysis
Inflammation	Complement proteins stimulate mast cells to release histamine

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Natural Killer (NK) Cells

• NK cells are a type of lymphocyte (but part of the innate immune system, not the adaptive system).
• They recognise and destroy virus-infected cells and cancer cells.
• They kill target cells by releasing perforin (which forms pores in the target cell membrane) and granzymes (which trigger apoptosis — programmed cell death).
• They do not require prior sensitisation or antigen presentation — hence they are "non-specific".

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Interferons

• Interferons are signalling proteins (cytokines) released by virus-infected cells.
• They "interfere" with viral replication by:
  • Signalling neighbouring cells to produce enzymes that inhibit viral replication.
  • Activating NK cells and macrophages.
  • Increasing MHC class I expression on cell surfaces, making infected cells more visible to cytotoxic T cells.

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Summary Table: Non-Specific Defences

Defence	Type	Location	Target
Skin	Physical barrier	External surface	All pathogens
Mucus and cilia	Physical/mechanical	Respiratory tract	Airborne pathogens
Stomach acid	Chemical	Stomach	Ingested pathogens
Lysozyme	Chemical (enzyme)	Tears, saliva, nasal secretions	Bacteria (peptidoglycan)
Phagocytosis	Cellular	Blood and tissues	All pathogens
Inflammation	Cellular/chemical	Site of infection	All pathogens
Complement	Chemical (proteins)	Blood and tissues	All pathogens
Interferons	Chemical (cytokines)	Released by infected cells	Viruses
NK cells	Cellular	Blood and tissues	Virus-infected cells, cancer cells

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Key Terms

Term	Definition
Phagocytosis	The process by which phagocytes engulf and destroy pathogens
Opsonisation	Coating a pathogen with molecules (antibodies, complement) that enhance phagocytosis
Antigen-presenting cell (APC)	A cell (e.g. macrophage) that displays pathogen antigens on its surface to activate T cells
Inflammation	A localised response to infection involving vasodilation, increased capillary permeability, and recruitment of phagocytes
Complement system	A group of plasma proteins that assist (complement) the immune response through opsonisation, chemotaxis, and lysis
Interferon	A cytokine released by virus-infected cells that inhibits viral replication in neighbouring cells

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Summary

• The non-specific immune response includes physical barriers (skin, mucus, acid) and internal defences (phagocytosis, inflammation, complement, interferons, NK cells).
• Phagocytosis involves chemotaxis, recognition, engulfment, digestion, and antigen presentation.
• The inflammatory response increases blood flow and capillary permeability to deliver immune cells to the infection site.
• Antigen presentation by macrophages is the critical bridge to the specific immune response.

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A-Level Deep Dive: Non-Specific Immune Response

Spec mapping

The Edexcel 9BI0 specification places the non-specific (innate) immune response within Topic 6: Immunity, Infection and Forensics, after pathogen biology and plant defences and before the specific (adaptive) response. The pedagogical sequencing is deliberate: lesson 1 supplies the pathogen classes the innate system must recognise; lesson 5 supplies the parallel innate system in plants (PRR signalling, ROS bursts, induced defence) that sharpens what is distinctive about animal innate immunity; lesson 7 begins the adaptive arm, primed by antigen presentation from the very phagocytes covered here. Synoptic links run to Topic 2 (phagosome–lysosome fusion uses SNARE-like membrane fusion machinery) and Topic 7 (vasodilation, increased vessel permeability and fluid leak in inflammation depend on the same arteriolar and capillary physiology covered in transport). Relevant statements concern physical and chemical barriers, phagocytosis, inflammation and innate priming of adaptive immunity (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).

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Worked example with full mark scheme

Question (8 marks):

A deep splinter wound contaminated with Staphylococcus aureus (Gram-positive) becomes red, hot, swollen and painful within minutes, with pus accumulating within hours.

(a) Describe the cascade that recruits neutrophils from the bloodstream to the wound and explain how a neutrophil engulfs and destroys the bacterium. (5)

(b) Explain how the same macrophage population also primes the adaptive immune response. (3)

Solution with mark scheme:

(a) Stage 1 — pattern recognition. Tissue-resident macrophages detect S. aureus via PRRs — principally TLRs — binding conserved PAMPs such as peptidoglycan and lipoteichoic acid. Recognition triggers cytokine release (IL-1, IL-6, TNF-$\alpha$α, chemokine IL-8).

M1 (AO1.1) — PAMPs recognised by PRRs/TLRs; cytokines released. Common error: "the macrophage detects bacteria" without naming receptor or PAMP.

Stage 2 — vascular response. Locally released histamine (from mast cells) and cytokines drive vasodilation and increase vessel-wall permeability, producing the four classical signs. Endothelial cells upregulate adhesion molecules (selectins, ICAM-1) that capture circulating neutrophils.

M1 (AO1.2) — vasodilation and permeability via histamine; endothelial adhesion molecules.

Stage 3 — chemotaxis and diapedesis. Neutrophils roll, adhere firmly, undergo diapedesis between endothelial cells, and follow the chemokine gradient (chemotaxis) to the wound within minutes.

M1 (AO1.2) — chemotaxis along chemokine gradient; diapedesis through vessel wall.

Stage 4 — opsonisation and engulfment. S. aureus is coated by complement C3b and antibodies — opsonisation — enhancing binding via neutrophil complement and Fc receptors. The neutrophil extends pseudopodia and engulfs the bacterium into a phagosome.

M1 (AO1.2) — opsonisation by C3b; pseudopodial engulfment; phagosome formation.

Stage 5 — phagolysosome and killing. The phagosome fuses with lysosomes to form a phagolysosome. Killing combines (i) acidic pH (~4.5), (ii) the NADPH-oxidase respiratory burst generating ROS, and (iii) hydrolytic enzymes (lysozyme, proteases). Spent neutrophils accumulate as pus.

A1 (AO3.1a) — three independent killing mechanisms (acid, ROS, hydrolases) earns the analytical mark.

(b) M1 (AO2.1) — the macrophage processes pathogen proteins into peptide fragments inside the phagolysosome.

M1 (AO2.1) — fragments are loaded onto MHC class II and displayed on the macrophage surface; the macrophage acts as an antigen-presenting cell (APC).

A1 (AO3.1a) — the MHC II–peptide complex is recognised by T helper cells (CD4$^+$+), which orchestrate the adaptive response. Antigen presentation is the bridge between innate destruction and adaptive memory.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Compare the roles of complement, interferons and NK cells in the non-specific immune response, and explain why each is needed despite functional overlap with phagocytosis.

Mark scheme decomposition by AO:

Marking point	AO	Credit-worthy content
1	AO1.1	States that complement is a $\sim$∼30-protein soluble cascade activated via three pathways (classical, alternative, lectin) converging on C3 cleavage, with three downstream functions: opsonisation (C3b), chemotaxis (C3a/C5a) and the membrane attack complex (MAC) that lyses bacteria by pore formation.
2	AO1.2	States that interferons are cytokines released by virus-infected cells that warn neighbours to upregulate antiviral enzymes (PKR, OAS/RNase L), inducing an antiviral state — defence specifically against intracellular replication that phagocytosis cannot reach.
3	AO1.2	States that NK cells are innate lymphocytes that destroy virus-infected and tumour cells via perforin and granzymes, recognising "missing self" (reduced MHC class I) — complementing interferon-mediated MHC I upregulation.
4	AO2.1	Explains that complement attacks pathogens outside host cells; interferons and NK cells together attack pathogens inside host cells — a division of labour phagocytosis alone cannot cover.
5	AO3.1a	Discusses functional overlap: complement opsonisation enhances phagocytosis; macrophages produce cytokines and respond to interferons. The arms are integrated, not redundant.
6	AO3.2a	Concludes the innate response is a multi-arm system whose overlap gives robust, broad-spectrum cover before the adaptive response arrives — selection has favoured layered defence over a single optimal mechanism.

Total: 6 marks (AO1 = 2, AO2 = 2, AO3 = 2). Specimen question modelled on the Edexcel 9BI0 paper format. Edexcel rewards candidates who contrast extracellular vs intracellular targeting rather than merely listing the four arms.

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Synoptic links

1. Lesson 1 (pathogen classes). PRRs detect different PAMP classes — TLR4 binds bacterial LPS, TLR2 peptidoglycan, TLR3 viral double-stranded RNA, TLR9 unmethylated bacterial CpG DNA, mannose-binding lectin recognises fungal mannans. Each PRR–PAMP pairing maps a pathogen class onto a specific innate signalling pathway.
2. Lesson 5 (plant innate immunity). Plant and animal innate immunity converged on the same logic: PRR-driven PAMP recognition, ROS bursts, induced defence chemistry. The deepest divergence is cellular versus chemical — animals deploy migratory phagocytes; plants, being sessile, rely on cell-autonomous chemistry. PRR convergence is a clean example of independent evolution onto the same molecular solution.
3. Lesson 7 (adaptive immunity). Macrophage antigen presentation via MHC class II to T helper cells is the molecular handover from innate to adaptive immunity. Without it the adaptive arm is never primed and memory never forms. The phagolysosome is therefore both a digestion compartment and the antigen-processing factory that supplies peptides for MHC II loading.
4. Topic 2 (membrane fusion). Phagosome–lysosome fusion uses SNARE-family membrane-fusion machinery identical in principle to synaptic vesicle fusion, secretion and endocytosis. Mycobacterium tuberculosis survives inside macrophages by blocking phagolysosomal fusion — a textbook example of how molecular cell biology determines disease outcome.
5. Topic 7 (cardiovascular). The visible signs of inflammation rely on cardiovascular physiology: histamine drives vasodilation of arterioles, and increases capillary permeability so that plasma proteins (complement, antibodies, fibrinogen) and phagocytes leak into tissue. Inflammation is an applied case of arteriolar and capillary control.
6. Complement as soluble cascade. Complement is the third arm parallel to phagocytes and lymphocytes — a $\sim$∼30-protein cascade with three activation routes (classical via antibody, alternative via spontaneous C3 hydrolysis, lectin via mannose-binding lectin) converging on C3 cleavage to C3a (chemoattractant) and C3b (opsonin), and downstream onto the C5b–C9 membrane attack complex. The zymogen-cascade architecture is shared with blood clotting.

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Mark-scheme literacy