A level-readyAll subjects · Biology · WBI14 (Unit 4)
The specific immune response: B cells and T cells
The specific immune response targets one antigen. Antigen-presenting cells, T helper cells, B cells and T killer cells work together, and memory cells give lasting protection.
Antigens and antigen presentation
An antigen is a molecule (usually a protein or glycoprotein) that triggers an immune response. Each pathogen carries antigens specific to it.
A macrophage engulfs a pathogen, digests it and displays its antigens on its surface, attached to MHC proteins. It is now an antigen-presenting cell (APC). Infected body cells also present viral antigens.
T cells and B cells
Each lymphocyte has receptors complementary to just one antigen.
- T helper cells: a T helper cell with complementary receptors binds the antigen on an APC. It is activated, divides by mitosis (clonal expansion) and releases cytokines that activate B cells, T killer cells and macrophages.
- T killer cells: activated T killer cells bind to body cells displaying the antigen (such as virus-infected cells) and release chemicals such as perforin that make holes in the membrane, killing the cell.
- B cells: a B cell whose antibody receptors bind the antigen takes it in and presents it. With cytokines from T helper cells it divides by mitosis into plasma cells, which secrete large amounts of antibody, and memory B cells.
- Memory T and B cells stay in the blood for years.
Antibodies and the secondary response
An antibody is a protein made of four polypeptide chains (two heavy, two light) held by disulfide bonds. It has a constant region and two identical variable regions forming two antigen-binding sites complementary to one antigen.
Antibodies cause agglutination (clumping pathogens together), neutralise toxins, and mark pathogens for phagocytes (opsonisation).
On a second exposure, memory cells respond at once: the secondary response is faster, bigger and longer-lasting, so the pathogen is destroyed before symptoms appear. This is immunity.
Worked example
Explain why the secondary immune response is faster than the primary response.
- During the primary response, memory B and T cells specific to the antigen are made.
- These remain in the blood in large numbers.
- On re-infection they recognise the antigen at once and divide rapidly into plasma cells and T killer cells.
Answer: Memory cells already exist, so clonal expansion and antibody production start immediately.
Check yourself
- How many antigen-binding sites does one antibody molecule (of the basic Y-shaped type) have?
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2. Two variable regions, one at the end of each arm.
- How many polypeptide chains make up one antibody molecule?
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4. Two heavy chains and two light chains.
- What is an antigen-presenting cell?
- A plasma cell secreting antibodies
- A cell such as a macrophage that displays a pathogen's antigens on its surface
- A red blood cell carrying oxygen
- A cell that produces histamine
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A cell such as a macrophage that displays a pathogen's antigens on its surface. APCs show antigens to T helper cells.
- What is the function of plasma cells?
- To engulf pathogens
- To kill infected cells with perforin
- To secrete large amounts of antibody
- To present antigens to B cells
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To secrete large amounts of antibody. Plasma cells are antibody factories.
- How do T killer cells destroy virus-infected cells?
- They bind to cells displaying the antigen and release perforin
- They secrete antibodies
- They engulf the virus particles in the blood
- They release histamine
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They bind to cells displaying the antigen and release perforin. Perforin makes holes in the infected cell.
- What do activated T helper cells release to stimulate B cells and T killer cells?
- Antibodies
- Lysozyme
- Cytokines
- Interferon only
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Cytokines. Cytokines are signalling molecules.
- How does agglutination help destroy bacteria?
- Antibodies clump bacteria together so phagocytes can engulf them more easily
- Antibodies dissolve the bacteria
- Bacteria are made to divide faster
- It stops blood clotting
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Antibodies clump bacteria together so phagocytes can engulf them more easily. Each antibody has two binding sites, linking pathogens together.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.