Specific Immune Response

Spec Mapping — OCR H420 Module 4.1.1 — Communicable diseases, content statement on the specific immune response: T and B lymphocytes, clonal selection, clonal expansion, primary and secondary responses, cell-mediated and humoral immunity (refer to the official OCR H420 specification document for exact wording). This lesson covers the adaptive arm of the immune system that builds on the antigen presentation of Lesson 7 and underlies the antibody biology of Lesson 9 and the vaccination strategy of Lesson 10.

The specific (adaptive) immune response is the body's targeted, learned response to particular pathogens. It is slower than the innate response to develop — taking ~7–14 days on first exposure to build to peak antibody concentration — but it is both specific (tailored to each pathogen, with single-amino-acid resolution) and has memory (persisting decades after the original encounter). The intellectual history is rich: Paul Ehrlich proposed in 1900 that cells carry "side chains" (receptors) that bind specific antigens — the "magic bullet" framework; Frank Macfarlane Burnet in 1957 proposed the clonal selection theory — that lymphocytes pre-exist with random receptor specificities and are selected by antigens, not instructed; Peter Doherty and Rolf Zinkernagel demonstrated MHC restriction of T-cell recognition (Nobel 1996); Susumu Tonegawa explained the somatic gene rearrangement (V(D)J recombination) that generates the receptor diversity (Nobel 1987). OCR specification 4.1.1 requires you to know T and B cell biology, clonal selection and expansion, and the difference between primary and secondary responses.

Key Definitions:

• Antigen — a molecule recognised by the immune system; usually a protein or glycoprotein on a pathogen surface.
• Epitope — the specific small region of an antigen that a single antibody or TCR recognises (usually 5–15 residues).
• Lymphocyte — a white blood cell of the adaptive immune system (T or B cell).
• Clonal selection — the process by which a lymphocyte with a receptor matching an antigen is selected from the pre-existing diverse repertoire for activation; antigens select, they do not instruct.
• Clonal expansion — the rapid mitotic proliferation of a selected lymphocyte clone to produce large numbers of identical effector and memory cells.
• Memory cell — a long-lived lymphocyte that persists after an infection and rapidly reactivates on re-exposure, mediating the secondary response.
• Cytokine — a small signalling protein (interleukins, interferons, TNF, chemokines) released by immune cells to communicate with each other.
• Apoptosis — programmed cell death; the mechanism by which cytotoxic T cells kill virus-infected target cells.

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Origin of Lymphocytes

Both T and B lymphocytes develop from stem cells in the bone marrow. They differ in where they mature:

• B cells mature in the bone marrow (B = bone marrow in mammals — historically "bursa of Fabricius" in birds).
• T cells mature in the thymus (T = thymus), a gland above the heart that shrinks after puberty.

During maturation, each lymphocyte expresses a unique antigen receptor on its surface — the T-cell receptor (TCR) on T cells, surface immunoglobulin on B cells. The repertoire is generated by V(D)J recombination, a process of random somatic gene rearrangement first elucidated by Susumu Tonegawa (Nobel 1987). Lymphocyte precursor cells carry V, D and J gene segments that are spliced together by the RAG1/RAG2 recombinase in random combinations, generating an estimated 10¹¹ different antibody specificities and a comparable diversity of TCRs.

This diversity comes at a cost: many of the random receptors will, by chance, recognise the body's own molecules. Central tolerance in the thymus (T cells) and bone marrow (B cells) deletes autoreactive cells before they emerge into the periphery. Cells that bind self-antigens too strongly during maturation undergo apoptosis (negative selection). Failures of central tolerance underlie autoimmune diseases such as type 1 diabetes (auto-reactive T cells attacking pancreatic β cells) and lupus (auto-antibodies against nuclear self-antigens).

Clonal selection — the central principle

flowchart TD
    A[Stem cell in bone marrow] --> B[V(D)J recombination]
    B --> C[Lymphocyte with unique random receptor]
    C --> D{Self-reactive?}
    D -->|Yes| E[Apoptosis: negative selection]
    D -->|No| F[Mature naive lymphocyte enters circulation]
    F --> G[Lymph node]
    G --> H{Antigen encounter?}
    H -->|No matching antigen| I[Cell circulates, eventually dies]
    H -->|Matching antigen| J[Clonal selection: this clone activated]
    J --> K[Clonal expansion by mitosis]
    K --> L[Effector cells: plasma B / cytotoxic T]
    K --> M[Memory cells: long-lived]

Burnet's clonal selection theory has three key tenets:

1. Each lymphocyte carries one and only one receptor specificity, determined before it ever meets an antigen.
2. The body contains an enormous pre-existing repertoire of lymphocytes with random specificities — many more than the number of antigens it will encounter in a lifetime.
3. Antigens select the few matching clones for activation and expansion — they do not instruct the receptor shape.

A common A-level error is to write "the body makes an antibody specific to the pathogen" as if the antigen instructed the antibody shape. Clonal selection is the opposite: the antibody already existed (as the receptor of a single B cell among millions), and the pathogen simply selected it for amplification.

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T Lymphocytes

T lymphocytes recognise antigens only when they are presented on MHC molecules by other cells. They come in several functional classes.

1. T Helper Cells (Th, CD4+)

• Bind to antigens presented on MHC class II by antigen-presenting cells (macrophages, dendritic cells, B cells).
• Release cytokines (interleukins) that activate B cells, killer T cells and macrophages.
• The "conductor" of the immune response — without Th cells, neither the humoral nor cell-mediated response works properly (this is why HIV is so devastating).

2. Cytotoxic / Killer T Cells (Tc, CD8⁺)

• Bind to antigens presented on MHC class I, which is expressed on all nucleated cells.
• MHC-I displays peptides derived from cytosolic proteins — so cells infected with viruses, intracellular bacteria or expressing mutated/tumour proteins display "abnormal" peptides on their MHC-I.
• Activated cytotoxic T cells bind such cells and release perforin (which polymerises to form pores in the target membrane) and granzymes (serine proteases that enter through the perforin pores and activate caspases, triggering apoptosis of the target).
• Cytotoxic T cells therefore kill the infected cell rather than the pathogen directly — a key principle that explains why much of viral and intracellular bacterial pathology is immunopathology, caused by the immune response rather than the pathogen itself.

3. Regulatory T Cells (Treg)

• Suppress immune responses once a pathogen has been cleared.
• Prevent autoimmune disease by damping down reactions against self-antigens.

4. Memory T Cells

• Long-lived cells that persist after the pathogen is cleared.
• Rapidly reactivated on subsequent exposure to the same antigen, giving a faster, larger response.

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B Lymphocytes

B lymphocytes are responsible for the humoral response — producing antibodies that circulate in blood and lymph.

1. B Effector / Plasma Cells

• Divide rapidly to form plasma cells.
• Plasma cells are antibody "factories", secreting up to 2,000 antibody molecules per second.
• Short-lived (a few days); each clone produces antibodies specific to a single antigen.

2. Memory B Cells

• Long-lived (years to decades).
• Do not secrete antibodies initially but remain ready to reactivate on re-exposure.

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Clonal Selection and Clonal Expansion

The core concept of the adaptive immune response is clonal selection: out of billions of lymphocytes with different receptors, the few whose receptors happen to match the invading antigen are selected for activation.

flowchart TD
    A[Antigen enters body] --> B[APC presents antigen on MHC II]
    B --> C[T helper cell with matching receptor binds]
    C --> D[Th cell releases cytokines]
    D --> E[B cell with matching receptor is activated]
    E --> F[Clonal expansion: rapid mitotic division]
    F --> G[Plasma cells secrete antibodies]
    F --> H[Memory B cells formed]
    D --> I[Killer T cells activated]
    I --> J[Clonal expansion of killer T cells]
    J --> K[Kill virus-infected cells]
    J --> L[Memory T cells formed]

Steps

1. Antigen detection. A pathogen is engulfed by an antigen-presenting cell (APC) and presented on MHC II.
2. T helper activation. A T helper cell with a matching receptor binds the antigen–MHC II complex. The APC releases interleukin-1; the T helper releases interleukin-2, activating itself and other lymphocytes.
3. B cell activation. B cells with receptors (surface antibodies) matching the same antigen bind it, engulf it, and present it on their own MHC II. T helper cells recognise this and release cytokines that activate the B cell.
4. Clonal expansion. Activated B cells divide rapidly by mitosis, producing a clone of identical cells.
5. Differentiation. Most of the clone become plasma cells secreting antibodies; a smaller fraction become memory B cells.
6. Cell-mediated arm. In parallel, cytotoxic T cells with receptors for the antigen on MHC I undergo their own clonal expansion and begin killing infected cells.

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The Primary and Secondary Responses

When the body meets a new pathogen for the first time, this whole process of clonal selection and expansion takes around 7–14 days before antibody levels peak — the primary response. The person is usually ill during this period.

On a second exposure to the same antigen — even months or years later — memory cells are already present in large numbers. They recognise the antigen immediately and undergo rapid clonal expansion. This secondary response is:

• Faster (days rather than weeks).
• Larger (higher peak antibody concentration).
• Longer-lasting.
• Typically prevents symptoms — the pathogen is cleared before it causes illness.

flowchart LR
    A[First exposure] --> B[Primary response: slow, small, IgM dominant]
    B --> C[Memory cells formed]
    C --> D[Second exposure]
    D --> E[Secondary response: fast, large, IgG dominant]
    E --> F[No symptoms]

You will see this illustrated by a classic graph:

Feature	Primary	Secondary
Time to peak antibody	~10–14 days	~3–5 days
Peak antibody concentration	Low	10–100× higher
Dominant antibody class	IgM	IgG
Symptoms	Usually present	Usually absent
Memory cells produced	Yes	More

This is the basis of immunological memory, and the rationale for vaccination — a deliberate, safe primary exposure so that the secondary response prevents illness on subsequent natural exposure.

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Cell-Mediated vs Humoral Immunity

The adaptive response has two arms:

Cell-mediated (cellular) response

• Mediated by T cells, especially cytotoxic T cells.
• Targets intracellular pathogens: viruses, intracellular bacteria, cancer cells.
• Works by killing infected cells directly.

Humoral response

• Mediated by B cells and the antibodies they produce.
• Targets extracellular pathogens: most bacteria, free viruses in body fluids.
• Works by neutralising, agglutinating and opsonising pathogens.

Both arms are coordinated by T helper cells.

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Comparing B and T Cells

Feature	B cells	T cells
Origin	Bone marrow stem cells	Bone marrow stem cells
Maturation site	Bone marrow	Thymus
Receptor	Surface immunoglobulin (BCR)	T-cell receptor (TCR)
Antigen recognition	Free or surface antigen, native conformation	Peptide presented on MHC
Effector form	Plasma cell (secretes antibody)	T helper (cytokines), cytotoxic T (kills target)
Memory form	Memory B cell	Memory T cell
Main target	Extracellular pathogens, toxins, free virus	Intracellular pathogens (viruses, intracellular bacteria), cancer cells
Arm of immunity	Humoral	Cell-mediated

Comparing Primary and Secondary Responses — Antibody-Time Curves