OCR GCSE Biology A: White blood cells, the immune system and vaccination (J247)

This study guide is aligned to the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018. It covers statements B6.3j, B6.3k, B6.3l and B6.3o of module B6.3, Monitoring and maintaining health. None of these four statements is in bold, so all of them are assessed in both Foundation and Higher Tier papers. Types of disease and how they spread, named infections, monoclonal antibodies, aseptic technique and drug testing, and non-communicable disease are taught on other pages.

Use this guide first, then the revision notes for fast recall and the practice questions to test yourself. Course pages: OCR GCSE Biology hub and printable checklist. To find weak spots, try the free 10-minute diagnostics. Blood and the circulatory system are covered on the exchange surfaces and circulatory system page, and resistant bacteria connect to natural selection.

What this unit covers

Statement	What you must be able to do	Tier
B6.3j	Explain how white blood cells and platelets are adapted to their defence functions in the blood.	Both
B6.3k	Describe the non-specific defence systems of the human body against pathogens.	Both
B6.3l	Explain the role of the immune system of the human body in defence against disease.	Both
B6.3o	Explain the use of vaccines and medicines in the prevention and treatment of disease, including antibiotics, antivirals and antiseptics.	Both

The mathematical outcomes for the module include translating between graphical and numerical forms and constructing and interpreting tables and charts. Expect antibody graphs and blood count tables.

B6.3j: Blood cells that defend the body

A pathogen is a microorganism that causes disease. Blood carries cells that find and deal with pathogens, and cell fragments that seal wounds. You need to link each structure to its job.

Phagocytes

Phagocytes (neutrophils are one example) are white blood cells that destroy pathogens by phagocytosis.

1. The phagocyte detects a pathogen and moves towards it.
2. It engulfs the pathogen, which ends up inside a vacuole in the cytoplasm.
3. Enzymes from lysosomes in the cytoplasm digest the pathogen.

Adaptations to link to function:

• A flexible, lobed nucleus and the ability to change shape let the cell squeeze out of capillaries and move through tissues to the site of infection.
• The cell surface membrane can fold around a pathogen to engulf it.
• Many lysosomes hold the digestive enzymes that break the pathogen down.

Lymphocytes

Lymphocytes are white blood cells that make antibodies. Their nucleus is large and takes up most of the cell. Each lymphocyte has receptors on its surface that fit one antigen only. When one meets a matching antigen it divides many times, and its daughter cells make antibodies with the same specific shape.

Platelets

Platelets are not whole cells. They are small fragments of larger cells made in the bone marrow, so they have no nucleus. When a blood vessel is cut, platelets stick to the damaged area and release chemicals. These start a series of reactions in which soluble fibrinogen is converted into insoluble fibrin. The fibrin forms a mesh of threads across the wound. Red blood cells are trapped in the mesh and a clot forms, which dries into a scab. The clot stops blood loss and stops pathogens entering. Platelets are adapted by being small, so they travel freely in the blood, and by releasing their clotting chemicals only at damaged tissue.

Worked example 1: a blood count

A healthy adult has 7000 white blood cells and 5 000 000 red blood cells per mm³ of blood. A patient with a bacterial infection has 15 500 white blood cells per mm³.

(a) Calculate the percentage increase in white blood cells.

increase = 15 500 - 7000 = 8500
percentage increase = 8500 / 7000 x 100 = 121.4... = 121 %

(b) Calculate how many red blood cells there are for each white blood cell in the healthy adult.

5 000 000 / 7000 = 714.3... = about 714

(c) Explain why the white blood cell count rises during infection. More lymphocytes are made and more phagocytes are produced, so that there are enough cells to engulf pathogens and make antibodies against them.

B6.3k: Non-specific defences

Non-specific defences work against any pathogen. They do not need to recognise a particular microbe. They have two jobs: to stop pathogens getting in, and to destroy those that do.

Defence	How it works
Skin	A continuous, tough barrier that most pathogens cannot cross.
Blood clotting and scabs	Seal cuts in the skin quickly (see platelets above).
Nose hairs and mucus	Hairs and sticky mucus in the nose and airways trap dust and microbes.
Cilia	Hair-like projections on cells lining the trachea and bronchi sweep mucus with trapped microbes up to the throat, and it is then swallowed.
Stomach acid	Hydrochloric acid in the stomach kills most pathogens that are swallowed with food or mucus.
Tears	Contain an enzyme, lysozyme, which breaks down the cell walls of some bacteria.
Phagocytes	Engulf and digest any pathogen that gets into the body.

The word non-specific is the key. A phagocyte will engulf many different microbes, and stomach acid kills most of what it meets. Compare this with the next section, where each response targets one type of pathogen.

B6.3l: The immune system

When a pathogen gets past the non-specific defences, the immune system gives a specific response.

Every pathogen has molecules on its surface called antigens. Each type of pathogen has its own antigens. This is the sequence you should be able to describe:

1. The pathogen enters the blood or tissues.
2. A lymphocyte with a receptor that matches the antigen binds to it.
3. That lymphocyte divides to form a clone of identical cells. Most become cells that secrete antibodies.
4. Antibodies are proteins whose shape is complementary to the antigen. They bind to the pathogen.
5. Antibodies can make pathogens clump together, mark them so phagocytes engulf them more easily, or neutralise toxins the pathogen has released (antitoxins).
6. Some of the clone stay in the blood as memory cells.

Memory cells are the reason you rarely catch the same disease twice. If the same antigen appears again, memory cells recognise it at once and divide quickly. You make antibodies faster, in greater amounts, and for longer. This is the secondary response. You usually destroy the pathogen before you feel ill, and you are immune.

The first time, the response is slower because few lymphocytes carry the matching receptor. You may become ill while the clone is built up. That is the primary response.

Worked example 2: reading an antibody graph

A person is exposed to an antigen on day 0 and again on day 28. Antibody concentration (arbitrary units) was measured.

Day	0	7	14	21	28	31	35	42
Antibody	0	2	10	6	3	25	90	75

(a) Describe the difference between the first and second responses.

• Peak of the first response is 10 units, at day 14. Peak of the second is 90 units, at day 35.
• After the first exposure it takes 14 days to reach the peak. After the second it takes 35 - 28 = 7 days.
• Antibodies are measured at 25 units only 3 days after the second exposure, but only 2 units 7 days after the first.

(b) Calculate how many times greater the second peak is.

90 / 10 = 9 times

(c) Explain the difference. Memory cells made after the first exposure recognise the antigen. They divide and make antibodies faster and in larger numbers than the first lymphocytes did.

B6.3o: Vaccines and medicines

Vaccination (prevention)

A vaccine contains a safe form of the antigen. It may be dead or inactive pathogens, a weakened pathogen, or parts of the pathogen that carry the antigens. It cannot cause the disease, but it still has the antigens.

1. The vaccine is injected.
2. Lymphocytes with matching receptors divide and make antibodies. This is the primary response, and it is harmless.
3. Memory cells remain in the blood.
4. If the real pathogen later infects you, memory cells give a rapid, large secondary response, and you do not become ill.

Vaccines protect before infection, so they are used for prevention. Some need booster doses to raise the level of memory cells again. Limits to remember: a vaccine is specific to one pathogen, so a measles vaccine does nothing for flu. Some pathogens, like flu viruses, change their antigens often, so the vaccine has to be updated. A few people have side effects such as a sore arm, and some vaccines do not give protection to everyone.

Medicines (treatment)

Medicines treat people who are already ill. Learn the three named in the specification.

• Antibiotics kill bacteria or stop them growing. They work on bacterial structures and processes, so they do not work on viruses. Different antibiotics work on different bacteria.
• Antivirals stop viruses replicating. Viruses live inside host cells and use the cell’s own machinery, so a medicine that harms the virus easily harms the host. This makes antivirals hard to develop, and each is designed for a particular virus (for example, aciclovir is used against herpes viruses).
• Antiseptics are applied to living tissue, such as skin or a wound, to kill microorganisms or prevent them growing, for example iodine solution or an alcohol hand gel. They are used to stop infection, not to cure one that is established.

Painkillers relieve symptoms but do not kill pathogens.

Antibiotic resistance

Bacteria vary. A mutation may make a bacterium resistant to an antibiotic. When the antibiotic is used, non-resistant bacteria die, and resistant bacteria survive and reproduce, so the population is now resistant. Using antibiotics only when needed, and finishing the course, slows this.

Worked example 3: how many survive?

A population of 4 000 000 bacteria is exposed to an antibiotic that kills 99.95 % of them.

percentage surviving = 100 - 99.95 = 0.05 %
survivors = 0.05 / 100 x 4 000 000 = 2000

If these 2000 are resistant, they reproduce with little competition and the next population is resistant.

Worked example 4: choosing a treatment

Situation	Best choice	Reason
Bacterial throat infection	Antibiotic	The pathogen is a bacterium.
Influenza virus infection	Antiviral, not antibiotic	Antibiotics do not act on viruses.
Graze on a knee	Antiseptic on the skin	Kills microbes on tissue before infection starts.
Child due for measles protection	Vaccine	Prevents the disease.

Common errors

• Saying antibiotics kill viruses, or that taking them cures a cold.
• Saying individual bacteria “become resistant” because of the antibiotic. The resistance was already there by mutation.
• Writing that vaccines “contain antibodies”. They contain antigens.
• Saying platelets are cells with a nucleus.
• Giving “kills pathogens” as an answer for a non-specific defence without saying how.
• Forgetting memory cells in an explanation of the second response.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), first assessment 2018, published by OCR. This page teaches statements B6.3j, B6.3k, B6.3l and B6.3o.