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

These revision notes are aligned to the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018. They cover statements B6.3j, B6.3k, B6.3l and B6.3o of module B6.3. None of these statements is in bold, so all are assessed in both Foundation and Higher Tier papers.

For full explanations and worked examples, read the study guide. Then test yourself with the practice questions. Course pages: OCR GCSE Biology hub and printable checklist. Not sure where to start? Take a free 10-minute diagnostic.

Key terms

Term	Meaning
Pathogen	A microorganism that causes disease.
Antigen	A molecule on the surface of a cell or pathogen that the immune system recognises.
Antibody	A protein made by lymphocytes with a shape complementary to one specific antigen.
Antitoxin	An antibody that neutralises a toxin.
Phagocytosis	Engulfing a pathogen and digesting it with enzymes.
Memory cell	A lymphocyte that stays in the blood after an infection and responds quickly to the same antigen.
Non-specific	Works against any pathogen, whatever its type.
Specific	Works against one antigen only.
Vaccine	A safe form of an antigen, given to produce memory cells before infection.

B6.3j: Blood cells and defence

Component	Structure	Function and link
Phagocyte	Flexible, changes shape, lobed nucleus, many lysosomes	Squeezes out of capillaries, engulfs the pathogen, enzymes from lysosomes digest it.
Lymphocyte	Large nucleus, little cytoplasm, surface receptors for one antigen	Divides when its antigen is met and makes antibodies.
Platelet	Small cell fragment, no nucleus, made in bone marrow	Releases chemicals at a wound so fibrinogen becomes fibrin and a clot forms.

Method in steps for phagocytosis:

1. Detect and move to the pathogen.
2. Engulf it into a vacuole.
3. Lysosome enzymes digest it.

Method in steps for clotting:

1. Vessel is cut, platelets stick to the damage.
2. Platelets release chemicals.
3. Soluble fibrinogen converted into insoluble fibrin threads.
4. Mesh traps red blood cells, forming a clot and then a scab.
5. Result: blood loss stopped, and pathogens kept out.

Worked reminder: percentage change in a blood count

White blood cells rise from 6200 to 9300 per mm³.

change = 9300 - 6200 = 3100
percentage change = 3100 / 6200 x 100 = 50 %

Platelets fall from 250 000 to 90 000 per mm³.

fall = 160 000
percentage fall = 160 000 / 250 000 x 100 = 64 %

Always divide by the original value. Write the unit or the percentage sign.

B6.3k: Non-specific defences

Non-specific means the same defence works whatever the pathogen is.

Barrier	Mechanism
Skin	Tough physical barrier.
Clot and scab	Seal breaks in the skin.
Nose hairs and mucus	Trap microbes in the airways.
Cilia	Sweep mucus up the trachea and bronchi to the throat to be swallowed.
Stomach acid	Hydrochloric acid kills most swallowed pathogens.
Tears	Lysozyme enzyme breaks down bacterial cell walls.
Phagocytes	Engulf and digest any pathogen inside the body.

Two groups to sort them into: those that stop pathogens entering (skin, mucus, cilia, nose hairs, tears, scabs) and those that destroy them (stomach acid, lysozyme, phagocytes).

B6.3l: The immune system

Specific response, step by step:

1. Pathogen antigens meet a lymphocyte with a matching receptor.
2. That lymphocyte divides to give a clone.
3. Clone cells secrete antibodies of the same specific shape.
4. Antibodies bind to antigens and can cause clumping, make phagocytosis easier, or neutralise toxins.
5. Memory cells remain.

Feature	Primary response	Secondary response
Trigger	First exposure to an antigen	Later exposure to the same antigen
Speed	Slow, few matching lymphocytes	Fast, memory cells already present
Antibody amount	Lower	Much higher
Duration	Shorter	Longer
Illness	Likely	Unlikely, because the pathogen is destroyed quickly

Must-know distinction: the immune system is specific, but phagocytes and the barriers are not. An antibody fits one antigen like a key fits a lock, so the antibody for one disease will not work on another.

Worked reminder: describing an antibody graph

Use three kinds of point and include figures:

1. Time lag before antibodies appear.
2. Height of the peak.
3. Time taken to reach the peak.

Example data: first exposure peak 12 units on day 10, second exposure peak 84 units, 5 days after exposure.

84 / 12 = 7 times higher

B6.3o: Vaccines and medicines

Item	What it is	Prevention or treatment	Works on
Vaccine	Dead, inactive or weakened pathogen, or its antigens	Prevention	One specific pathogen
Antibiotic	Kills bacteria or stops them growing	Treatment	Bacteria only
Antiviral	Stops virus replicating inside cells	Treatment	A particular virus
Antiseptic	Applied to skin or wounds to kill microbes	Prevents infection	Many microorganisms, on living tissue
Painkiller	Reduces pain	Symptom relief only	Does not affect the pathogen

How a vaccine works:

1. Safe antigens injected, so there is no disease.
2. Lymphocytes with matching receptors divide and make antibodies.
3. Memory cells are left in the blood.
4. Real infection later: fast, large secondary response, so no illness.

Points to weigh up for vaccines:

• Benefits: protection without having the illness, and fewer cases of the disease.
• Risks: possible mild side effects such as a sore arm, and rarely a stronger reaction.
• Limits: not everyone is protected, boosters may be needed, and viruses that change antigens (like flu) need new vaccines.

Antibiotic resistance in order:

1. Random mutation gives some bacteria resistance.
2. The antibiotic kills the non-resistant bacteria.
3. Resistant bacteria survive and reproduce.
4. The population is now mostly resistant.

Slowed by using antibiotics only when needed and completing the course.

Worked reminder: survivors

A population of 3 000 000 bacteria, antibiotic kills 99.9 %.

0.1 / 100 x 3 000 000 = 3000 survivors

Must-know distinctions

Often confused	Difference
Antigen and antibody	An antigen is on the pathogen. An antibody is made by a lymphocyte and binds to it.
Antibody and antitoxin	An antitoxin is an antibody that neutralises a toxin released by a pathogen.
Vaccine and antibiotic	A vaccine prevents disease by building memory cells. An antibiotic treats a bacterial infection that has already started.
Antiseptic and antibiotic	An antiseptic goes on living tissue from outside. An antibiotic is taken to act inside the body.
Phagocyte and lymphocyte	A phagocyte engulfs and digests. A lymphocyte makes antibodies.
Primary and secondary	Primary is the first meeting with an antigen. Secondary is every meeting after, driven by memory cells.

Worked reminder: effect of a vaccination programme

Cases of a disease fall from 5000 a year to 400 a year after a programme starts.

fall = 5000 - 400 = 4600
percentage decrease = 4600 / 5000 x 100 = 92 %

Use the figures in the answer, then say why: more people have memory cells, so fewer infections start.

Command words

• Describe: say what happens or what a graph shows, using figures.
• Explain: give a reason, with the biology that links cause to effect. For a structure, state the feature, then the way it helps the function.
• Suggest: use what you know to apply to a new situation, such as why a patient is prescribed an antiseptic and not an antibiotic.
• Evaluate: weigh benefits against risks and finish with a judgement.

Quick self-test

1. What is a pathogen?
2. Name the process by which phagocytes destroy pathogens.
3. Why do platelets not count as whole cells?
4. State what is converted into what when blood clots.
5. Give two non-specific defences in the airways.
6. What does the stomach do that is non-specific?
7. Which part of a pathogen does an antibody bind to?
8. Why is a vaccine safe even though it contains antigens?
9. Why do antibiotics not work for influenza?
10. White blood cells rise from 8000 to 11 000 per mm³. Calculate the percentage increase.
11. What type of medicine would be used on a graze?
12. State one reason why the flu vaccine is updated.

Answers

1. A microorganism that causes disease.
2. Phagocytosis.
3. They are fragments of larger cells and have no nucleus.
4. Soluble fibrinogen into insoluble fibrin.
5. Any two of: mucus, cilia, nose hairs.
6. It produces hydrochloric acid that kills most pathogens that are swallowed.
7. An antigen.
8. The pathogens in it are dead, inactive or weakened, or only parts of the pathogen, so it cannot cause the disease.
9. Influenza is caused by a virus, and antibiotics act on bacteria.
10. 3000 / 8000 x 100 = 37.5 %.
11. An antiseptic.
12. Flu viruses change their antigens, so memory cells for last year’s strain may not recognise the new one.

Where marks are usually lost

• Writing that phagocytes “eat” bacteria without naming engulfing and digesting by enzymes.
• Saying white blood cells “attack” or “fight” without stating the mechanism.
• Calling platelets cells, or saying they “stick” without mentioning fibrin.
• Writing that vaccines contain antibodies, rather than antigens.
• Leaving memory cells out of any explanation of a faster second response.
• Describing a graph without using figures from it.
• Dividing by the wrong value in a percentage change.
• Saying antibiotics treat viral disease or that bacteria “get used to” the drug.
• Giving an answer for a non-specific defence that only says “kills pathogens”, with no mechanism.

Official syllabus

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