Specific Immune Response

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 several days on first exposure — but it is both specific (tailored to each pathogen) and has memory, providing long-lasting protection. OCR specification 4.1.1 (f) requires you to describe the structure and role of T lymphocytes and B lymphocytes, clonal selection and expansion, and the difference between primary and secondary responses.

Key Definitions:

• Antigen — a molecule that the immune system recognises; usually a protein or glycoprotein on a pathogen surface.
• Lymphocyte — a type of white blood cell that mediates the adaptive immune response (T and B cells).
• Clonal selection — the process by which a lymphocyte with a receptor matching an antigen is selected for activation.
• Clonal expansion — the mass proliferation of a selected lymphocyte clone to produce large numbers of effector cells.
• Memory cells — long-lived lymphocytes that remain after an infection, providing immunity.

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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 repertoire is generated by random gene rearrangement, producing billions of different specificities. Cells that would react against the body's own antigens are deleted, leaving only those that will respond to foreign antigens (self-tolerance).

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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 present on all nucleated cells.
• MHC I displays peptides from inside the cell — so cells infected with viruses or carrying mutated proteins show up as "abnormal".
• Killer T cells bind to these cells and release perforin, which forms pores in the target membrane, and granzymes, which trigger apoptosis.

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.