How does the specific immune response work, how does vaccination give immunity, and why are antibiotics losing their power?
4.1.1 The immune response: the structure and function of antibodies; the roles of B and T lymphocytes in the humoral and cell-mediated responses; the primary and secondary responses and the role of memory cells; the principles of vaccination and herd immunity; the differences between active, passive, natural and artificial immunity; and the development of antibiotic resistance.
A focused answer to the OCR H420 4.1.1 dot point on the specific immune response. Covers antibody structure, B and T lymphocytes, the primary and secondary responses, memory cells, vaccination and herd immunity, the four types of immunity, and how antibiotic resistance evolves.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
OCR wants you to describe antibody structure and function, explain the roles of B and T lymphocytes in the humoral and cell-mediated responses, contrast the primary and secondary responses and the role of memory cells, explain vaccination and herd immunity, distinguish the four types of immunity, and explain how antibiotic resistance evolves.
The answer
Antibodies
An antibody is a globular protein (immunoglobulin) with quaternary structure: two heavy and two light polypeptide chains arranged in a Y shape, held by disulfide bonds. The variable region at the tips has a specific shape complementary to one antigen, so each antibody binds only one antigen, forming an antigen-antibody complex. The constant region is the same within a class of antibody and determines its function (for example binding to phagocytes). Antibodies work by agglutination (clumping pathogens), acting as opsonins (marking pathogens for phagocytosis), and neutralising toxins.
B and T lymphocytes
The specific response has two arms:
- Cell-mediated response (T lymphocytes). A macrophage presents antigen; T helper cells with complementary receptors are activated and release interleukins (cytokines) that stimulate the appropriate B cells, T cells and phagocytes. T killer (cytotoxic) cells destroy body cells infected with the pathogen.
- Humoral response (B lymphocytes). A B cell whose antibody matches the antigen is selected (clonal selection) and, stimulated by T helper cells, divides (clonal expansion) into plasma cells (which secrete large amounts of antibody) and memory cells.
Primary and secondary responses
- The primary response (first exposure) is slow because the correct lymphocytes must be selected and undergo clonal expansion; antibody levels rise slowly and the person may feel ill. Memory cells are left behind.
- The secondary response (re-exposure to the same antigen) is faster, larger and longer-lasting because memory cells recognise the antigen at once and divide rapidly into plasma cells, usually destroying the pathogen before symptoms appear.
Vaccination and herd immunity
A vaccine introduces antigens (a dead or weakened pathogen, or its antigens) to trigger a primary response and produce memory cells without causing the disease, so a later real infection meets a rapid secondary response. Herd immunity occurs when a high proportion of the population is immune, so the pathogen cannot spread easily; this protects unvaccinated individuals (for example the very young or immunocompromised) because infected people contact fewer susceptible hosts.
Types of immunity
| Type | How antibodies arise | Memory cells? | Example |
|---|---|---|---|
| Natural active | Body makes its own after catching the disease | Yes (long-lasting) | Recovering from measles |
| Artificial active | Body makes its own after vaccination | Yes (long-lasting) | A measles vaccine |
| Natural passive | Ready-made antibodies received naturally | No (short-lived) | Antibodies via placenta or breast milk |
| Artificial passive | Ready-made antibodies injected | No (short-lived) | Anti-venom or antibody injection |
Active immunity makes its own antibodies and memory cells (slow but long-lasting); passive immunity uses ready-made antibodies (immediate but short-lived, no memory).
Antibiotic resistance
Antibiotics kill bacteria or inhibit their growth but do not affect viruses. Resistance evolves by natural selection: a random mutation gives a bacterium resistance; when the antibiotic is present it acts as a selection pressure, killing non-resistant bacteria while resistant bacteria survive and reproduce, passing on the resistance allele (often on plasmids). Over generations the allele frequency increases. Overuse and misuse accelerate this, which is why finishing prescribed courses and reserving antibiotics for bacterial infections matter.
Examples in context
Example 1. Maternal antibodies in breast milk. A baby receives ready-made antibodies through the placenta and breast milk (natural passive immunity), giving immediate but short-lived protection while its own immune system develops.
Example 2. MRSA. Methicillin-resistant Staphylococcus aureus is a hospital "superbug" that evolved resistance to many antibiotics through repeated selection, illustrating why antibiotic stewardship and hygiene are vital.
Try this
Q1. Describe the role of T helper cells in the specific immune response. [2 marks]
- Cue. They are activated by presented antigen and release interleukins (cytokines) that stimulate the appropriate B cells, T killer cells and phagocytes.
Q2. Explain why passive immunity does not give long-term protection. [2 marks]
- Cue. The antibodies are ready-made and are not produced by the person's own lymphocytes, so no memory cells are formed; the antibodies are broken down and not replaced.
Q3. State what is meant by herd immunity. [1 mark]
- Cue. When a high proportion of a population is immune, so the pathogen cannot spread easily and unvaccinated individuals are protected.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20186 marksExplain why the secondary immune response to a pathogen is faster and larger than the primary response.Show worked answer →
Contrast the two responses through the action of memory cells.
In the primary response, the antigen is encountered for the first time. The correct B and T lymphocytes must be selected (clonal selection) and then divide (clonal expansion), which is slow, so antibody production is delayed and the person may feel ill. After the antigen is cleared, memory cells remain.
In the secondary response, the same antigen is met again. The memory cells recognise it immediately and divide rapidly into many plasma cells, so antibodies are produced faster, in greater quantity and for longer. The pathogen is usually destroyed before symptoms appear, so the person is immune.
Markers reward clonal selection and expansion being slow in the primary response, and memory cells producing a faster, larger, longer response on re-exposure.
OCR H420/02 20224 marksExplain how antibiotic resistance arises in a population of bacteria and why the overuse of antibiotics increases it.Show worked answer →
Frame it as natural selection acting on random variation.
A random mutation in a bacterium can give resistance to an antibiotic; this variation exists before the antibiotic is used. When the antibiotic is present it acts as a selection pressure: non-resistant bacteria are killed, but the resistant bacteria survive and reproduce, passing on the resistance allele (including by plasmid transfer).
Over generations the frequency of the resistance allele increases, so more of the population is resistant. Overuse and misuse (not finishing courses, using antibiotics for viral infections) increases the selection pressure and the chance that resistant strains spread.
Markers reward random mutation, selection pressure, survival and reproduction of resistant bacteria, and increasing allele frequency.
Related dot points
- 4.1.1 Communicable diseases: the range of pathogens (bacteria, viruses, fungi and protoctists) and the communicable diseases they cause in animals and plants; the means of transmission; the primary non-specific defences of plants and animals; and the role of phagocytes in the non-specific immune response.
A focused answer to the OCR H420 4.1.1 dot point on communicable diseases. Covers the four pathogen groups and example diseases, means of transmission, the primary non-specific defences of plants and animals, and the role of phagocytes in non-specific immunity.
- 4.2.2 Evolution: the process of evolution by natural selection acting on variation; the role of mutation in generating variation; the types of natural selection (directional, stabilising and disruptive); the evidence for evolution from fossils, comparative anatomy and molecular biology; and examples such as antibiotic resistance and industrial melanism.
A focused answer to the OCR H420 4.2.2 dot point on evolution. Covers natural selection acting on variation, mutation as the source of variation, directional, stabilising and disruptive selection, the evidence for evolution, and examples such as antibiotic resistance and peppered moths.
- 4.2.2 Classification and evolutionary relationships: the binomial system and the taxonomic hierarchy; the five kingdoms and the three-domain classification; the meaning of phylogeny; and how molecular evidence (DNA base sequences, amino acid sequences) and other evidence are used to clarify evolutionary relationships.
A focused answer to the OCR H420 4.2.2 dot point on classification. Covers the binomial system and taxonomic hierarchy, the five kingdoms and the three-domain system, the meaning of phylogeny, and how molecular and other evidence is used to establish evolutionary relationships.
- 2.1.2 Biological molecules: the structure and function of triglycerides and phospholipids; the structure of amino acids, the formation of peptide bonds and the four levels of protein structure; the structure of nucleotides, DNA and RNA; the biochemical tests for lipids (emulsion test) and proteins (biuret test).
A focused answer to the OCR H420 2.1.2 dot point on lipids, proteins and nucleic acids. Covers triglycerides and phospholipids, amino acids and the four levels of protein structure, nucleotide and DNA and RNA structure, and the emulsion and biuret tests.
- 6.1.3 Manipulating genomes: the principles of DNA sequencing, the polymerase chain reaction (PCR) and gel electrophoresis; the use of restriction enzymes and ligase to produce recombinant DNA in genetic engineering; the principles of gene editing; and the use of DNA profiling.
A focused answer to the OCR H420 6.1.3 dot point on manipulating genomes. Covers DNA sequencing, the polymerase chain reaction and gel electrophoresis, restriction enzymes and ligase in genetic engineering, the principles of gene editing, and DNA profiling.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)