Immunity — A-Level Biology Study Guide

For: A-Level Biology candidates sitting A-Level Biology.

Covers: Innate vs adaptive immunity, B cell and antibody primary/secondary responses, helper and cytotoxic T cell function, active/passive vaccines and herd immunity, and monoclonal antibody production and real-world applications.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the A-Level Biology style for educational use. They are not reproductions of past Cambridge International examination papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official Cambridge mark schemes for grading conventions.

1. What Is Immunity?

Immunity is the set of interconnected biological defences that protect an organism from infectious pathogens (viruses, bacteria, fungi, parasites) and abnormal host cells (e.g. pre-cancerous or cancerous cells) to prevent acute and chronic disease. It is also referred to as immune system function or pathogen resistance in some syllabus resources. This is a high-weight core topic for A-Level Biology, tested across all papers: 1-3 multiple choice marks in Paper 1, 2-5 structured marks in Paper 2, and 3-8 extended response marks in Paper 4 in most exam series.

2. Innate vs adaptive immunity

The immune system is split into two interconnected branches with distinct roles:

• Innate immunity: Non-specific defences present from birth that act identically every time they are exposed to a foreign substance, with no memory of prior exposures. Key components include:

1. Physical barriers: Skin, mucus membranes, cilia in the respiratory tract
2. Chemical barriers: Lysozyme in tears/saliva, stomach acid, antimicrobial peptides in sweat
3. Cellular defences: Phagocytes (neutrophils, macrophages) that engulf and destroy foreign material, natural killer cells that kill abnormal host cells
4. Inflammatory response: Histamine release causes local swelling, heat and redness to recruit immune cells to injury sites

• Adaptive immunity: Specific, acquired defences that develop after exposure to a specific foreign antigen (a molecule that triggers an immune response), with long-term memory of prior exposures. Key components include B lymphocytes, T lymphocytes and antibodies.

Worked Example

If you scrape your knee on contaminated soil:

1. Innate response activates within minutes: Histamine causes inflammation, neutrophils migrate to the wound and phagocytose 70-90% of invading bacteria within 2 hours.
2. If remaining bacteria evade the innate response, adaptive immunity activates 3-7 days later, producing defences targeted only at the specific antigens on the invading bacteria.

The core difference between the two branches can be quantified: innate responses have the same speed and magnitude every time you are exposed to a pathogen, while adaptive secondary responses are ~10x faster and produce ~10x more defensive molecules than primary responses.

3. B cells and antibodies — primary and secondary response

B cells are a type of lymphocyte that mature in the bone marrow, with unique antigen receptors on their surface that only bind to one specific antigen. When an antigen binds to a matching B cell receptor, and the B cell receives a co-stimulating signal from a helper T cell, it differentiates into two cell types:

1. Plasma cells: Short-lived cells that produce large volumes of antibodies (Y-shaped glycoproteins with two identical antigen binding sites)
2. Memory B cells: Long-lived cells that remain in circulation for months to years, storing a template of the target antigen for future exposures

Antibodies have three core functions:

• Neutralisation: Bind to pathogens to block them from entering host cells
• Opsonisation: Mark pathogens to make them easier for phagocytes to identify and engulf
• Agglutination: Clump multiple pathogens together to reduce mobility and speed up phagocytosis

Primary vs Secondary Immune Response

When you are exposed to an antigen for the first time, you trigger a primary response:

• Lag phase of 3-7 days before antibody levels rise
• Peak antibody concentration of ~10 arbitrary units (AU) at 7-10 days
• Antibody levels drop to near-zero within 4-6 weeks as short-lived plasma cells die off

When you are exposed to the same antigen a second time, you trigger a secondary response:

• Lag phase of only 1-2 days, as memory B cells activate immediately
• Peak antibody concentration of ~100 AU, per the relationship: $$\text{Secondary response peak [antibody]}\approx 10\times \text{Primary response peak [antibody]}$$
• Antibody levels remain elevated for months to years, often preventing symptoms entirely even if you are exposed to the pathogen.

Worked Example

A 5-year-old is infected with chickenpox for the first time: they develop symptoms 10 days after exposure, and are sick for 7 days while the primary response activates. Memory B cells for chickenpox antigens remain in their body for life. At age 25, they are exposed to chickenpox again: the secondary response eliminates the virus within 48 hours, so they never develop symptoms.

4. T cells — helper and cytotoxic

T cells are lymphocytes that mature in the thymus, with T cell receptors that only bind to antigens presented on the surface of host cells, attached to major histocompatibility complex (MHC) molecules. There are two core types of T cells with distinct roles:

1. Helper T cells (CD4+): Activated when they bind to antigens presented by antigen-presenting cells (e.g. macrophages that have engulfed and broken down a pathogen). When activated, they release cytokine signalling molecules that:

• Stimulate B cell differentiation into plasma cells and memory B cells
• Activate cytotoxic T cells
• Increase phagocyte activity

2. Cytotoxic T cells (CD8+): Bind to infected or abnormal host cells (e.g. virus-infected cells, cancer cells) that present foreign antigens on their MHC molecules. They release perforin (a protein that creates holes in the host cell membrane) and granzymes (enzymes that enter the cell and trigger programmed cell death, or apoptosis), preventing the pathogen from replicating and spreading to other cells.

Worked Example

HIV specifically targets and destroys helper T cells. As helper T cell counts drop below 200 cells per mm³ of blood, B cells and cytotoxic T cells cannot receive the activation signals they need to respond to pathogens. This leaves the body vulnerable to even harmless opportunistic infections, the defining feature of AIDS.

5. Vaccines — active and passive, herd immunity

Vaccines are preparations of weakened (attenuated), killed, or fragmented pathogens, or their inactivated toxins, that trigger an immune response without causing the full disease. Immunity from vaccines is classified into two broad categories:

Active Immunity

Active immunity develops when your own immune system produces memory cells in response to an antigen, so it is long-lasting (often lifelong). It has two subtypes:

• Natural active immunity: Develops after natural infection with a pathogen
• Artificial active immunity: Develops after vaccination

Passive Immunity

Passive immunity develops when you receive antibodies produced by another organism, rather than making them yourself. No memory cells are produced, so it is short-lived (lasting only weeks to months until the antibodies are broken down by the body). It has two subtypes:

• Natural passive immunity: Antibodies passed from mother to foetus across the placenta, or to an infant in breast milk
• Artificial passive immunity: Injection of pre-made antibodies, e.g. anti-venom for snake bites, rabies antibodies given after exposure to the virus

Herd Immunity

Herd immunity occurs when a high enough proportion of a population is immune to a pathogen (via vaccination or natural infection) that the pathogen cannot spread easily between people. The herd immunity threshold varies by pathogen, based on how contagious it is: ~95% for highly contagious measles, ~85% for polio. When the threshold is met, even people who cannot be vaccinated (infants, immunocompromised people, people with severe vaccine allergies) are protected, because they are very unlikely to come into contact with an infected person.

Worked Example

In 2022, measles vaccination rates in a region dropped from 96% to 81%. The basic reproduction number R (number of people one infected person will pass the virus to) rose from 0.8 (below 1, so outbreaks die out) to 3.2 (above 1, so outbreaks spread rapidly). 12 infants too young to receive the measles vaccine were infected in the subsequent outbreak, a direct result of falling below the herd immunity threshold.

6. Monoclonal antibodies — production and use

Monoclonal antibodies (mAbs) are identical antibodies produced from a single clone of plasma cells, specific to one single antigen. They are produced via the hybridoma technique, which is a frequent exam question:

1. Immunise a laboratory mouse with the target antigen, so it produces B cells that make antibodies against the antigen
2. Extract antibody-producing B cells from the mouse's spleen
3. Fuse the B cells with myeloma cells (cancerous plasma cells that divide indefinitely in culture) to create hybridoma cells: these combine the B cell's ability to produce antibodies with the myeloma cell's ability to replicate endlessly
4. Screen the hybridoma cells to identify the clone that produces the desired antibody, then grow the clone in large bioreactors to produce mass quantities of identical monoclonal antibodies

Common Uses of Monoclonal Antibodies

1. Diagnostics: Pregnancy tests (detect hCG hormone in urine), COVID-19 lateral flow tests, blood typing for transfusions
2. Therapeutics: Cancer treatment (mAbs bind to specific antigens on cancer cells, marking them for destruction by the immune system), treatment of autoimmune diseases (e.g. rheumatoid arthritis, mAbs block inflammatory cytokine signals)
3. Research: Purify specific proteins from mixtures, detect specific antigens in tissue samples

Worked Example

A home pregnancy test uses two types of monoclonal antibodies: one type binds to hCG and is linked to a blue dye, the second type binds to a different site on hCG and is fixed to the test line on the stick. If hCG is present in urine, it binds to both antibodies, so the blue dye accumulates at the test line, producing a visible positive result.

7. Common Pitfalls (and how to avoid them)

• Wrong move: Stating that innate immunity has memory of prior pathogen exposures. Why: Students mix up core features of the two immune branches. Correct move: Memory is the defining feature of adaptive immunity; innate responses are identical every time you encounter a pathogen.
• Wrong move: Confusing B and T cell functions, e.g. stating T cells produce antibodies or B cells kill infected cells. Why: Students mix up lymphocyte roles. Correct move: Use the mnemonic: B cells make Bodies (antibodies), T cells handle Targeting (coordinate responses or kill infected cells).
• Wrong move: Describing passive immunity as long-lasting. Why: Students assume all immunity is permanent. Correct move: Passive immunity only lasts as long as the injected antibodies remain in your body (weeks to months), because no memory cells are produced.
• Wrong move: Forgetting to mention vulnerable unvaccinated groups when explaining herd immunity. Why: Students only describe herd immunity as "stopping spread" without linking to its public health purpose. Correct move: Always note that herd immunity protects people who cannot receive vaccines (immunocompromised, infants, people with severe allergies) as well as the general population.
• Wrong move: Mixing up the steps of monoclonal antibody production, e.g. fusing B cells with red blood cells instead of myeloma cells. Why: Students forget the purpose of myeloma cells. Correct move: Myeloma cells are cancerous, so they divide indefinitely; hybridomas combine the B cell's antibody production ability with the myeloma's endless replication.

8. Practice Questions (A-Level Biology Style)

Question 1 (Paper 1 Multiple Choice)

Which of the following is a feature of adaptive immunity only? A) Presence of phagocytes B) Specificity to a single antigen C) Immediate response to pathogen exposure D) Lysozyme secretion in tears

Worked Solution: Correct answer = B. Explanation: A is incorrect: phagocytes are part of the innate immune system. C is incorrect: adaptive immunity has a 3-7 day lag phase, while innate responses are immediate. D is incorrect: lysozyme is an innate chemical barrier. Only adaptive immune defences have receptors specific to one single antigen.

Question 2 (Paper 2 Structured, 3 marks)

A hiker is given an injection of pre-made rabies antibodies immediately after being bitten by a stray dog. (a) Name the type of immunity this provides. (1 mark) (b) Explain why this immunity will only protect the hiker for 2-3 months. (2 marks)

Worked Solution: (a) Artificial passive immunity (1 mark) (b) Passive immunity does not stimulate the production of memory B or T cells (1 mark). The injected antibodies are gradually broken down by the body over time, so no long-term protection remains (1 mark).

Question 3 (Paper 4 Extended Response, 6 marks)

(a) Describe the role of helper T cells in the adaptive immune response. (3 marks) (b) Explain why the secondary immune response is faster and produces higher antibody levels than the primary response. (3 marks)

Worked Solution: (a) Helper T cells are activated when they bind to antigens presented by antigen-presenting cells such as macrophages (1 mark). They release cytokine signalling molecules that stimulate B cells to differentiate into plasma cells and memory B cells (1 mark), and also activate cytotoxic T cells and increase phagocyte activity (1 mark). (b) During the primary response, long-lived memory B cells specific to the target antigen are produced and remain in circulation (1 mark). On re-exposure to the same antigen, these memory cells bind the antigen and differentiate into antibody-producing plasma cells immediately, eliminating the long lag phase of the primary response (1 mark). Many more plasma cells are produced in the secondary response, leading to a 10x higher peak antibody concentration than the primary response (1 mark).

9. Quick Reference Cheatsheet

10. What's Next

This immunity topic connects directly to multiple later sections of the A-Level Biology syllabus, including the infectious disease unit (where you will learn about pathogens that evade immune defences, such as HIV, tuberculosis and cholera) and the cell signalling and specialised cell sections (since cytokine signalling and lymphocyte activation are core examples of cell communication pathways). A strong understanding of immunity is also required to answer high-weight extended response questions on public health interventions, which appear on almost every Paper 4 exam series.

If you have questions about any part of this guide, from interpreting antibody response graphs to memorising monoclonal antibody production steps, you can ask Ollie at any time for personalised explanations, extra practice questions, or step-by-step walkthroughs of past exam problems. Head to the homepage, to get started with your customised study plan for A-Level Biology.

Aligned with the Cambridge International AS & A Level Biology 9700 syllabus. OwlsAi is not affiliated with Cambridge Assessment International Education.