Immunity & Disease – A-Level Biology Revision Notes
Complete revision notes on phagocytosis, the cellular and humoral response, antibodies, vaccination, monoclonal antibodies and immunity types. Written by a former examiner and Immunology graduate, with the exact mark scheme language you need to earn full marks – and a board-by-board breakdown of what your specification requires.
Last updated: June 2026
Why Immunity Is a Topic I Take Personally
Immunity is my subject. My degree is a BSc (Hons) in Immunology from King’s College London, and I later sat on the Teachers’ Advisory Board for the Wellcome Trust’s Big Picture issue on the immune system. So when I tell you where students lose marks on this topic, it is not from skimming a textbook – it is from years of teaching it, examining it, and knowing the science behind it.
Here is the truth about immunity at A-Level: the biology is not actually that hard. What costs students marks is imprecise language. Write that an antibody “kills” a pathogen, muddle the cellular and humoral responses, or call a memory cell a “memory antibody”, and the marks vanish even when you clearly understand the idea. Examiners mark to a scheme, and the scheme rewards exact terms used in the right order.
On this page I take you through the entire immune response in the sequence examiners want to see it, flag the precise phrases that earn and lose marks, and then – crucially – show you exactly what your exam board requires. That last part matters more here than on almost any other topic, because immunity is examined very differently across the boards: for some it is compulsory core content, and for WJEC and Eduqas it is an optional topic that not every student even studies.
Key Terminology – The Words That Earn Marks
Get these definitions exactly right before you attempt any immunity question. Examiners have specific accept and reject criteria for every one of them.
The Body’s Defences – Non-Specific and Specific
Your body defends itself in two broad ways. Non-specific (innate) defences respond to any pathogen in the same way and act immediately. Specific (adaptive) defences are targeted to a particular antigen, take longer to develop, and produce lasting immunological memory.
Non-specific defences
- Physical and chemical barriers: skin, mucous membranes, ciliated epithelium, stomach acid (HCl), and lysozyme in tears and saliva that hydrolyses bacterial cell walls.
- Blood clotting and wound repair: platelets trigger a cascade producing fibrin, which seals the wound and prevents pathogen entry (named explicitly by OCR A).
- Inflammation: mast cells release histamine, causing vasodilation and increased capillary permeability, bringing more white blood cells to the site.
- Phagocytosis: the key non-specific cellular defence, covered in detail below.
Phagocytosis – The Non-Specific Cellular Response
Phagocytosis is carried out by phagocytes such as neutrophils and macrophages. It destroys pathogens directly and provides the vital link to the specific response by displaying antigens. Examiners typically award 4–5 marks for a full description, in this order:
The Cell-Mediated Response – T Lymphocytes
The cell-mediated (cellular) response involves T lymphocytes and acts mainly against the body’s own cells that have been altered – for example cells infected by a virus, cancer cells, or transplanted cells. It is triggered by antigen-presenting cells.
The Humoral Response – B Lymphocytes and Antibodies
The humoral response involves B lymphocytes and produces antibodies that act against antigens and pathogens in the body fluids (the “humours” – blood and tissue fluid).
Antibody structure
An antibody is a Y-shaped protein made of four polypeptide chains (two heavy, two light) held by disulfide bonds. The tips of the Y are the variable regions – their specific shape forms the antigen-binding sites. The rest is the constant region, the same in all antibodies of a class.
Primary and Secondary Responses, and Types of Immunity
The first time you meet an antigen, the primary response is slow because very few B cells have the complementary receptor – it takes time to select and clone them. Symptoms usually appear. After the infection, memory cells remain in the blood.
On a second exposure to the same antigen, the secondary response is faster and stronger: memory cells divide rapidly and plasma cells produce antibodies almost immediately, often destroying the pathogen before symptoms appear. This is the basis of long-term immunity and of vaccination.
Active vs passive immunity
| Feature | Active immunity | Passive immunity |
|---|---|---|
| How antibodies arise | Your own immune system makes them after meeting an antigen | Ready-made antibodies are introduced from outside |
| Examples | Infection (natural); vaccination (artificial) | Across the placenta and in breast milk (natural); antibody injection, e.g. anti-tetanus or anti-rabies (artificial) |
| Memory cells made? | Yes | No |
| Speed of protection | Slow to develop | Immediate |
| Duration | Long-lasting | Short-lived (antibodies are broken down) |
HIV, Antibiotics and Monoclonal Antibodies
HIV and AIDS (AQA core; Edexcel)
HIV (human immunodeficiency virus) infects and replicates inside helper T cells. As helper T cells are destroyed, the whole immune response is undermined – because helper T cells coordinate both the cellular and humoral responses. When numbers fall too low, the person develops AIDS and cannot fight off infections that a healthy immune system would handle.
Monoclonal antibodies (AQA core)
Monoclonal antibodies are identical antibodies produced from a single clone of B/plasma cells, all specific to one antigen. AQA requires their use in:
- Targeting medication: attaching a therapeutic drug to an antibody so it is delivered directly to specific cells (e.g. cancer cells), reducing side effects.
- Medical diagnosis: including the ELISA test, which uses antibodies with an attached enzyme to detect the presence and quantity of a specific antigen or antibody (used in disease testing and pregnancy tests).
Disease and Antibiotics – The WJEC/Eduqas Option
For WJEC and Eduqas students taking the “Immunology and Disease” option, the disease and antibiotic content carries a large share of the marks – and is exactly where I see candidates under-prepare. You need:
- Key terms: pathogenic, infectious, carrier, disease reservoir, endemic, epidemic, pandemic, vector, toxin, antigenic types – defined precisely.
- Named diseases analysed by: type of organism, source of infection, tissue affected, mode of transmission, prevention, control and treatment.
- Antibiotics: bacteriostatic vs bactericidal; broad vs narrow spectrum; the modes of action of penicillin (inhibits cell-wall synthesis) and tetracycline (inhibits protein synthesis); why the Gram-negative cell wall protects bacteria from many antibiotics and immune defences.
- Antibiotic resistance: how overuse of antibiotics has driven the spread of resistance, and the implications.
- Ethics of vaccination programmes and the differing effectiveness of immunisation against different diseases.
Exam Board Comparison – What Your Board Requires
This is the table no other revision site gives you. Immunity is examined very differently across the boards – use this to revise what your specification actually demands, and skip what it does not.
| Requirement | AQA | OCR A | OCR B | Edexcel A | WJEC / Eduqas |
|---|---|---|---|---|---|
| Status of topic | Core | Core | Core | Core | Optional (A2) |
| Phagocytosis | ✔ | ✔ | ✔ | ✔ | ✔ |
| Cellular & humoral response | ✔ | ✔ | ✔ | ✔ | ✔ |
| Plant defences | ❌ | ✔ | ❌ | ❌ | ❌ |
| Interferon named | ❌ | ❌ | ❌ | ✔ | ❌ |
| HIV / AIDS | ✔ | Via disease | Via disease | ✔ | Via disease |
| Monoclonal antibodies & ELISA | ✔ | ❌ | ❌ | ❌ | ❌ |
| Active/passive immunity | ✔ | ✔ | ✔ | ✔ | ✔ |
| Antibiotics & resistance detail | Limited | ✔ | ✔ | ✔ | ✔ |
| Named-disease analysis | ❌ | ✔ | ✔ | ✔ | ✔ |
Always check the current specification for your exam series, as boards update content periodically.
Immunity Is My Specialist Subject
My degree is in Immunology, I have examined this topic for WJEC/Eduqas and Edexcel, and I advised the Wellcome Trust on their immune-system teaching issue. If immunity feels like a jumble of cells and responses, I can make it click – taught to your exact exam board.
Tyrone John • CBiol MRSB • BSc Immunology • Former Examiner • 25+ Years Teaching
Book a Free Consultation8 Common Mistakes from Examiner Reports
These are the immunity errors I see again and again, both as an examiner and as a tutor. Every one of them costs marks.
| # | The mistake | The correction |
|---|---|---|
| 1 | “Antibodies kill / attack the pathogen” | Antibodies bind antigens to form a complex; destruction is by agglutination and phagocytosis. |
| 2 | Confusing antigen and antibody | Antigen = the foreign molecule that triggers the response. Antibody = the protein made against it. They are opposites, not synonyms. |
| 3 | Muddling cellular and humoral responses | Cellular = T cells acting on body cells. Humoral = B cells making antibodies against antigens in body fluids. |
| 4 | “The lysosome engulfs the pathogen” | The phagocyte engulfs the pathogen into a phagosome; the lysosome then fuses and releases lysozymes. |
| 5 | Saying memory cells “produce antibodies” | Memory cells do not secrete antibody. On re-exposure they divide and differentiate into plasma cells, which then secrete antibody. |
| 6 | Clonal selection vs clonal expansion mixed up | Selection = the right lymphocyte being chosen by the antigen. Expansion = that lymphocyte then dividing by mitosis. |
| 7 | “Vaccines give you the disease” | A vaccine introduces a safe form of the antigen, triggering a primary response and memory cells without causing the disease. |
| 8 | Active vs passive immunity confused | Active = your own cells make antibodies; long-lasting, memory cells formed. Passive = ready-made antibodies given; immediate but short-lived, no memory cells. |
Frequently Asked Questions – Immunity & Disease
It depends on your exam board. For AQA, OCR A, OCR B, Edexcel A and Edexcel B, immunity and the immune system are compulsory core topics. For WJEC and Eduqas, “Immunology and Disease” is an optional A2 topic, chosen as one of three options in the second exam. If you are on WJEC or Eduqas, confirm with your teacher that your class is studying this option before revising it in depth.
The cell-mediated response involves T lymphocytes and acts mainly on the body’s own cells that have been altered, for example by a virus, or on cancer and transplanted cells. It includes helper T cells coordinating the response and cytotoxic T cells destroying infected cells. The humoral response involves B lymphocytes, clonal selection and plasma cells that secrete antibodies against antigens and pathogens in the body fluids. The two are linked because helper T cells stimulate the B cells.
Active immunity is produced when your own immune system makes antibodies after meeting an antigen, either through infection (natural) or vaccination (artificial). It is long-lasting because memory cells are formed. Passive immunity is when ready-made antibodies are introduced from outside, for example across the placenta and in breast milk (natural) or by injection such as anti-tetanus (artificial). It works immediately but is short-lived, because no memory cells are made and the antibodies are eventually broken down.
Antibiotics target structures and processes found only in bacteria, such as cell-wall synthesis, bacterial ribosomes and bacterial enzymes. Viruses do not have these features and reproduce inside host cells using the host’s own machinery, so there is nothing for the antibiotic to act on. This is why antibiotics are useless against viral infections such as colds, flu and HIV.
A vaccine introduces a safe form of the antigen (for example a dead or weakened pathogen, or just the antigen itself). This triggers a primary immune response, so B cells undergo clonal selection and form plasma cells and, crucially, memory cells, without the person developing the disease. If the real pathogen is met later, the memory cells trigger a rapid, strong secondary response that destroys it before symptoms appear.
Memory cells are long-lived lymphocytes that remain in the blood after an infection or vaccination. They do not secrete antibodies themselves. Instead, if the same antigen is encountered again, memory B cells divide rapidly and differentiate into plasma cells that produce antibodies much faster and in greater quantity than during the primary response. This faster, stronger secondary response is the basis of long-term immunity.
For a Level 3 (5 to 6 mark) answer, work through the response in order and use precise terms: a phagocyte engulfs the pathogen and presents its antigens; a helper T cell with a complementary receptor is activated (clonal selection) and divides by mitosis; it stimulates B cells; the specific B cell divides and differentiates into plasma cells and memory cells; plasma cells secrete antibodies that bind antigens to form antigen-antibody complexes, causing agglutination and phagocytosis. Mention memory cells giving a faster secondary response. Never say antibodies “kill” the pathogen.