The Specific Immune Response: B Cells and T Cells

The specific (adaptive) immune response is activated when pathogens breach the non-specific defences. It involves lymphocytes — B cells and T cells — that recognise specific antigens and mount a targeted response. This lesson covers the roles of B cells and T cells, antigen presentation, clonal selection, and the cell-mediated and humoral immune responses for the Edexcel A-Level Biology (9BI0) specification.

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Key Concepts

Antigens

An antigen is any molecule (usually a protein or glycoprotein) that is recognised by the immune system and can trigger an immune response.

Type	Description	Example
Foreign antigens	Molecules on the surface of pathogens, toxins, or foreign cells	Bacterial cell wall proteins, viral capsid proteins, pollen proteins
Self-antigens	Molecules on the surface of the body's own cells	MHC (Major Histocompatibility Complex) proteins — unique to each individual

Self vs Non-Self Recognition

• The immune system distinguishes self (body's own cells, marked by MHC proteins) from non-self (foreign cells, marked by foreign antigens).
• Autoimmune diseases occur when the immune system mistakenly attacks self-antigens (e.g. Type 1 diabetes, rheumatoid arthritis).
• During development, T cells that react to self-antigens are destroyed in the thymus (clonal deletion), ensuring self-tolerance.

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Lymphocytes: B Cells and T Cells

Both B cells and T cells are produced from stem cells in the bone marrow but mature in different locations:

Feature	B cells	T cells
Maturation site	Bone marrow	Thymus
Receptors	B cell receptors (BCRs) — membrane-bound antibodies	T cell receptors (TCRs)
Function	Produce antibodies (humoral immunity)	Cell-mediated immunity (kill infected cells, activate B cells)
Antigen recognition	Can recognise free antigens directly	Recognise antigens only when presented on MHC proteins by APCs

Clonal Selection and Expansion

The body produces millions of different B cells and T cells, each with a unique receptor that recognises one specific antigen. This diversity is generated by genetic recombination during lymphocyte development.

When a pathogen enters the body:

1. Clonal selection: Only the lymphocyte(s) with receptors complementary to the pathogen's antigen are selected (activated).
2. Clonal expansion: The selected lymphocyte divides rapidly by mitosis, producing a large population of identical cells (a clone).
3. Differentiation: The clones differentiate into effector cells (which fight the pathogen) and memory cells (which provide long-term immunity).

Exam Tip: Clonal selection is a key concept. Each lymphocyte is specific to one antigen. Out of millions of lymphocytes, only those with the correct receptor are activated. This explains why the primary immune response takes several days — the correct clone must be found, expanded, and differentiated.

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The Cell-Mediated Immune Response (T Cells)

T cells are central to the cell-mediated response. They are activated when an antigen-presenting cell (APC) — typically a macrophage or dendritic cell — presents a processed antigen on its MHC class II protein.

Types of T Cells

T cell type	Function
T helper cells (Th, CD4⁺)	Recognise antigen-MHC II complexes on APCs. Release cytokines that: activate B cells (humoral response), activate cytotoxic T cells, enhance phagocyte activity
Cytotoxic T cells (Tc, CD8⁺)	Recognise antigen-MHC I complexes on infected body cells. Kill infected cells by releasing perforin (forms pores) and granzymes (trigger apoptosis)
Regulatory T cells (Treg)	Suppress the immune response after the pathogen has been cleared; prevent autoimmunity
Memory T cells	Long-lived cells that persist after infection; provide rapid secondary response upon re-exposure

Activation of T Helper Cells

1. A macrophage phagocytoses a pathogen and processes its antigens.
2. The macrophage presents the antigen on its MHC class II protein.
3. A T helper cell with the complementary T cell receptor (TCR) binds to the antigen-MHC II complex.
4. The T helper cell is activated and undergoes clonal expansion.
5. Activated T helper cells release cytokines (interleukins) that:
   • Stimulate B cells to divide and differentiate into plasma cells.
   • Stimulate cytotoxic T cells to divide and kill infected cells.
   • Enhance macrophage activity.

Killing of Infected Cells by Cytotoxic T Cells

1. A virus-infected cell displays viral antigens on its MHC class I protein.
2. A cytotoxic T cell with the complementary TCR binds to the antigen-MHC I complex.
3. The cytotoxic T cell releases perforin, which inserts into the target cell membrane, forming pores.
4. Granzymes enter through the pores and activate caspases (enzymes that trigger apoptosis).
5. The infected cell undergoes programmed cell death and is removed by phagocytes.

Exam Tip: HIV attacks T helper cells (CD4⁺), which explains why it is so devastating — without T helper cells, neither the cell-mediated nor the humoral immune response can be properly activated.

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The Humoral Immune Response (B Cells)

The humoral response involves B cells producing antibodies that circulate in the blood and body fluids (humour = fluid).

Activation of B Cells

1. A B cell binds a specific antigen via its B cell receptor (BCR) — a membrane-bound antibody.
2. The B cell internalises the antigen, processes it, and presents it on MHC class II.
3. A T helper cell recognises the antigen-MHC II complex on the B cell and releases cytokines.
4. These cytokines stimulate the B cell to undergo clonal expansion.
5. The clone differentiates into:
   • Plasma cells — effector cells that secrete large quantities of antibodies (up to 2,000 per second per cell).
   • Memory B cells — long-lived cells that persist in the body and provide rapid antibody production on re-exposure.

The following diagram summarises the humoral immune response from antigen recognition to antibody action:

graph TD
    A["Antigen binds to<br/>B cell receptor"] --> B["B cell activated<br/>(+ helper T cell)"]
    B --> C["Clonal Expansion"]
    C --> D["Plasma Cells<br/>(secrete antibodies)"]
    C --> E["Memory B Cells<br/>(long-lived)"]
    D --> F["Antibodies bind<br/>to antigens"]
    F --> G["Agglutination /<br/>Neutralisation"]

Important Note

Most B cells require T helper cell activation to fully differentiate. This is called T-dependent activation and is the most common pathway for protein antigens.

Some antigens (e.g. polysaccharide antigens) can activate B cells without T cell help (T-independent activation), but this produces a weaker response with limited memory.

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Primary and Secondary Immune Responses

Feature	Primary response	Secondary response
Timing	Slow (7–14 days to peak)	Rapid (1–3 days to peak)
Antibody levels	Lower peak	Much higher peak
Main antibody	IgM (initial), then IgG	Predominantly IgG
Cells involved	Naive B and T cells	Memory B and T cells
Why faster?	N/A	Memory cells are already present in large numbers; they recognise the antigen immediately and undergo rapid clonal expansion

The secondary response is the basis of vaccination and explains why you rarely get the same disease twice.

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The Role of MHC Proteins

MHC type	Found on	Presents antigens to	Function
MHC class I	All nucleated body cells	Cytotoxic T cells (CD8⁺)	Displays intracellular antigens (e.g. viral proteins produced inside the cell)
MHC class II	APCs only (macrophages, dendritic cells, B cells)	T helper cells (CD4⁺)	Displays extracellular antigens that have been phagocytosed and processed

Exam Tip: MHC I is on all nucleated cells (not red blood cells, which lack a nucleus). MHC II is only on antigen-presenting cells. This distinction is important for explaining which T cells respond and why.

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Key Terms

Term	Definition
Antigen	A molecule (usually a protein) that triggers a specific immune response
Clonal selection	The activation of a specific lymphocyte whose receptor matches the invading antigen
Clonal expansion	The rapid division (mitosis) of an activated lymphocyte to form a clone
Plasma cell	An effector B cell that secretes antibodies at a high rate
Memory cell	A long-lived lymphocyte that enables a faster, stronger secondary response
Cytokine	A signalling protein released by immune cells that regulates immune responses
Apoptosis	Programmed cell death, triggered by cytotoxic T cells or granzymes

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Summary

• B cells and T cells are the key players in the specific immune response.
• Clonal selection and expansion produce effector cells and memory cells.
• T helper cells coordinate the immune response by releasing cytokines.
• Cytotoxic T cells kill virus-infected cells via perforin and granzymes.
• B cells differentiate into plasma cells (antibody production) and memory B cells.
• The secondary response is faster and stronger due to memory cells.

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A-Level Deep Dive: The Specific Immune Response

Spec mapping

The Edexcel 9BI0 specification places the specific (adaptive) immune response within Topic 6: Immunity, Infection and Forensics, between the innate response and antibodies/vaccination. Lesson 6 (innate immunity) ended with MHC class II antigen presentation — the molecular handover that opens this lesson. Lesson 8 (antibodies) unpacks the antibody structure and effector mechanisms (opsonisation, agglutination, complement activation) downstream of the plasma cells generated here. Lesson 9 (vaccination) exploits the memory-cell component to drive priming without live pathogen exposure. Synoptic links extend to Topic 1 (antibody quaternary structure: two heavy + two light chains, hinge region, Fab and Fc split) and Topic 8 (receptor diversity from V(D)J recombination; autoimmune disease as failure of central tolerance). Relevant statements concern lymphocyte development, MHC-restricted antigen presentation, clonal selection and expansion, the cell-mediated and humoral arms, primary and secondary responses and immunological memory (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 child is exposed to Bordetella pertussis (whooping cough) for the first time. The bacterium is engulfed by a respiratory dendritic cell, and within ten days the child develops detectable serum antibodies and a cytotoxic T-cell response.

(a) Describe the cascade by which dendritic-cell antigen presentation activates a T helper cell, including the two-signal rule that prevents accidental activation. (4)

(b) Explain how the activated T helper cell drives the parallel humoral and cell-mediated arms, and account for the ten-day delay. (4)

Solution with mark scheme:

(a) Stage 1 — antigen processing and MHC II loading. The dendritic cell phagocytoses B. pertussis, lysosomal proteases cleave bacterial proteins into peptide fragments (~12–25 aa), and peptides are loaded onto MHC class II in an endosomal compartment, then trafficked to the surface.

M1 (AO1.1) — peptide loaded onto MHC class II, displayed at surface.

Stage 2 — TCR–MHC engagement (signal one). A naive CD4$^+$+ T helper with a complementary TCR binds the MHC II–peptide complex; the CD4 co-receptor stabilises the contact.

M1 (AO1.2) — TCR–MHC II engagement with CD4 stabilisation.

Stage 3 — co-stimulation (signal two). PRR engagement on the dendritic cell upregulates B7 (CD80/CD86), which binds CD28 on the T helper. Both signals required.

M1 (AO1.2) — CD28–B7 co-stimulation; the two-signal rule prevents accidental self-activation (signal one alone induces anergy).

Stage 4 — clonal expansion. Activated helpers upregulate IL-2 and IL-2 receptor; autocrine signalling drives proliferation; daughter clones differentiate into Th1, Th2 and memory subsets.

A1 (AO3.1a) — IL-2-driven autocrine proliferation; Th1/Th2/memory differentiation.

(b) M1 (AO2.1) — Humoral arm. Th2-polarised helpers release IL-4, IL-5, IL-6 that stimulate a B cell whose BCR has bound B. pertussis antigen. The B cell re-presents the same antigen on MHC II, the T helper recognises it (linked recognition), and the B cell undergoes clonal expansion into plasma cells and memory B cells.

M1 (AO2.1) — Cell-mediated arm. Th1-polarised helpers secrete IL-2 and IFN-$\gamma$γ that drive CD8$^+$+ cytotoxic T-cell proliferation. CTLs recognise B. pertussis peptides on MHC class I of infected epithelial cells and kill them by perforin/granzyme apoptosis.

M1 (AO1.2) — naming perforin and granzymes on the cell-mediated side; plasma cells and memory B cells on the humoral side.

A1 (AO3.1a) — the ten-day delay reflects clonal selection mathematics: ~1 in $10^6$106 naive lymphocytes match any antigen; that rare clone must be located, two-signal-activated and undergo ~10–15 rounds of mitosis. Slowness is the price of specificity — and exactly the lag vaccination abolishes by pre-establishing memory clones.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Compare and contrast the activation, effector mechanisms and final targets of CD4$^+$+ T helper cells and CD8$^+$+ cytotoxic T cells, and explain why HIV infection of CD4$^+$+ cells collapses both arms of adaptive immunity.

Mark scheme decomposition by AO:

Marking point	AO	Credit-worthy content
1	AO1.1	States that CD4$^+$+ T helper cells recognise antigen on MHC class II displayed by professional APCs (macrophages, dendritic cells, B cells), require two-signal activation (TCR–MHC II + CD28–B7), and act by secreting cytokines (IL-2, IL-4, IL-6, IFN-$\gamma$γ) that orchestrate other immune cells.
2	AO1.2	States that CD8$^+$+ cytotoxic T cells recognise antigen on MHC class I (present on all nucleated cells), are licensed by Th-derived IL-2, and kill infected target cells directly via perforin (pore formation) and granzymes (caspase-mediated apoptosis).
3	AO2.1	Contrasts the targets: T helpers act on immune cells (B cells, cytotoxic T cells, macrophages) — they coordinate; cytotoxic T cells act on any infected nucleated cell — they execute. T helpers do not kill; cytotoxic T cells do not coordinate.
4	AO2.1	Explains MHC-class targeting logic: MHC II presents extracellular antigens (engulfed by APCs) to CD4$^+$+ helpers; MHC I presents intracellular antigens (synthesised inside any cell, e.g. viral proteins) to CD8$^+$+ killers. The two classes report on different compartments.
5	AO3.1a	Explains the HIV paradox: depleting CD4$^+$+ T helpers simultaneously cripples the humoral arm (B cells lose linked-recognition cytokine help, cannot class-switch fully to IgG) and the cell-mediated arm (CD8$^+$+ cells lose IL-2 licensing). One cell type lost; both arms collapse.
6	AO3.2a	Concludes that the T helper is the central regulator — adaptive immunity is hub-and-spoke, not parallel-redundant, which is why a single CD4$^+$+ deficit (HIV, CD40L hyper-IgM syndrome) is catastrophic where a CD8$^+$+ deficit is not.

Total: 6 marks (AO1 = 2, AO2 = 2, AO3 = 2). Specimen question modelled on the Edexcel 9BI0 paper format. Edexcel rewards candidates who use HIV not just as a stock example but as the evidence for T-helper centrality — the disease demonstrates the architecture.

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Synoptic links