Blood, immunity and clotting – IGCSE Biology exam questions with mark scheme

1Grade 1–35 marks

The diagram shows a stained blood smear seen through a light microscope.

(a) Name the structures labelled A and D. [2]

(b) Cells B and C are both types of white blood cell. Name cell B and cell C. [2]

(a) A: red blood cell; accept erythrocyte; ignore 'blood cell' unqualified(1)
(a) D: platelet(s); accept thrombocyte; reject 'small cell'(1)
(b) B: phagocyte; accept neutrophil / macrophage; ignore 'white blood cell' (given in the question)(1)
(b) C: lymphocyte; reject lymph(1)
(c) plasma; reject serum / water / lymph(1)
Answer: (a) A red blood cell, D platelet (b) B phagocyte, C lymphocyte (c) plasma
Examiner tip: Use the nucleus to tell the white cells apart: a lobed nucleus means phagocyte, a large round nucleus filling most of the cell means lymphocyte.

2Grade 1–35 marks

Plasma transports many substances around the body.

(a) Complete the table. [4]

Substance carried in plasma	Carried from	Carried to
carbon dioxide	respiring cells	(i) ..........
urea	(ii) ..........	kidneys
(iii) .......... (a product of digestion)	small intestine	body cells
(iv) .......... (a hormone)	pancreas	liver

(b) During exercise, muscles release a lot of heat energy. Describe how plasma helps to stop the muscles overheating. [1]

(i) lungs; accept alveoli; ignore 'heart'(1)
(ii) liver(1)
(iii) glucose; accept amino acids / fatty acids / glycerol / named vitamin or mineral ion; reject starch / protein / fat(1)
(iv) insulin; accept glucagon(1)
(b) plasma absorbs heat from the muscles and carries it to other parts of the body / to the skin (where it is lost); accept distributes heat around the body; ignore 'plasma cools the muscles' unqualified(1)
Answer: (a) (i) lungs (ii) liver (iii) glucose / amino acids (iv) insulin (b) plasma carries heat away from the muscles to the rest of the body / skin
Examiner tip: Urea is made in the liver, not the kidneys; the kidneys only remove it from the blood.

3Grade 8–95 marks

A technician estimates the number of red blood cells in a sample of blood.

The blood is diluted 200 times with a salt solution.
A drop of the diluted blood is placed on a special counting slide. Each square on the slide holds a volume of liquid $0.2\,\text{mm} \times 0.2\,\text{mm} \times 0.1\,\text{mm}$.
The technician counts the red blood cells in 5 squares.

Square	1	2	3	4	5
Number of red blood cells	92	88	95	90	85

(a) Calculate the number of red blood cells in 1 mm³ of the undiluted blood. Give your answer in standard form. [4]

(b) Give one reason why the technician counted the cells in 5 squares rather than in 1 square. [1]

(a) mean number per square = 90(1)
(a) volume of one square = 0.004 mm³; accept 4 × 10⁻³ mm³(1)
(a) 90 ÷ 0.004 = 22 500 (cells per mm³ of diluted blood); allow ecf from incorrect mean or volume(1)
(a) 22 500 × 200 = 4.5 × 10⁶ (cells per mm³); allow ecf; correct answer not in standard form (4 500 000) scores 3 marks only(1)
(b) to calculate a mean / to make the estimate more representative of the whole sample; accept cells may not be evenly spread across the slide; accept to reduce the effect of an anomalous count; ignore 'to make it fair' / 'more accurate' unqualified(1)
Answer: (a) 4.5 × 10⁶ red blood cells per mm³ (b) gives a mean that is more representative, as cells are not evenly spread
Examiner tip: Students often forget to multiply by the dilution factor, giving an answer 200 times too small.

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4Grade 6–74 marks

Frog red blood cells are oval and contain a nucleus. Human red blood cells are biconcave discs with no nucleus.

Explain how the structure of human red blood cells makes them efficient at transporting oxygen. [4]

biconcave (shape) gives a large surface area to volume ratio; accept large surface area; ignore 'large' cell(1)
(so) oxygen diffuses in and out quickly / faster diffusion of oxygen; accept thin (centre) gives a short diffusion distance(1)
no nucleus so more space for haemoglobin; reject 'no nucleus so more space for oxygen' (oxygen is carried by haemoglobin)(1)
haemoglobin combines with / binds to oxygen (to form oxyhaemoglobin) in the lungs and releases it at the tissues; accept haemoglobin carries oxygen(1)
Answer: Biconcave shape gives a large surface area to volume ratio for fast diffusion of oxygen; no nucleus leaves more room for haemoglobin, which combines with oxygen to form oxyhaemoglobin.
Examiner tip: For each feature, say what it does: 'no nucleus' alone scores nothing until you link it to more haemoglobin.

5Grade 4–55 marks

A group of climbers spent 28 days at high altitude, where the air contains less oxygen. Their mean red blood cell count was measured every 7 days. The graph shows the results.

(a) Calculate the percentage increase in mean red blood cell count between day 0 and day 28. [2]

(b) Explain how this change helps the climbers to remain active at high altitude. [2]

(a) (6.0 − 5.0) ÷ 5.0 (× 100)(1)
(a) 20 (%); correct answer with no working scores 2(1)
(b) more red blood cells so more haemoglobin (in the blood); accept more cells to carry oxygen(1)
(b) so more oxygen can be carried to the muscles / cells for (aerobic) respiration; accept to release more energy; ignore 'more oxygen breathed in'(1)
(c) they still have more red blood cells / higher red blood cell count at sea level, so more oxygen reaches their muscles (during the race); accept improved performance / less anaerobic respiration linked to more oxygen carried(1)
Answer: (a) 20% (b) more red blood cells, so more haemoglobin to carry oxygen to cells for aerobic respiration (c) the extra red blood cells remain for the race, so more oxygen is delivered to the muscles
Examiner tip: Percentage change is always (new − original) ÷ original; dividing by 6.0 instead of 5.0 gives the wrong answer (16.7%).

6Grade 4–55 marks

Bacteria have entered a cut on a person's finger.

(a) Describe how a phagocyte destroys one of these bacteria. [3]

(b) Lymphocytes also help to defend the body. They produce antibodies.

Explain why the antibodies made against these bacteria would not protect the person against a different type of bacterium. [2]

(a) phagocyte moves towards / is attracted to the bacterium; accept squeezes out of capillaries to the infected area(1)
(a) engulfs / ingests / surrounds the bacterium (into a vacuole); accept phagocytosis; reject 'eats'(1)
(a) digests / breaks down the bacterium using enzymes(1)
(b) antibodies are specific (to one pathogen / antigen); accept antibodies have a shape complementary to the antigens on that bacterium(1)
(b) a different bacterium has different antigens / a different shape, so the antibodies cannot bind to it; reject 'antibodies kill the bacteria'(1)
Answer: (a) moves to the bacterium, engulfs it into a vacuole, digests it with enzymes (b) antibodies are specific; the other bacterium has different antigens so the antibodies will not fit / bind
Examiner tip: Phagocytes engulf and digest; lymphocytes make antibodies. Students who say lymphocytes engulf bacteria lose the marks.

7Grade 6–7Triple only5 marks

A blood test shows that a patient has 40 000 platelets per mm³ of blood. The normal range is 150 000 to 400 000 platelets per mm³. The patient bruises easily and small cuts bleed for a long time.

(a) Explain how platelets are involved in forming a blood clot at a cut. [3]

(b) Give two reasons why it is important that a clot forms quickly at a cut in the skin. [2]

(a) platelets release chemicals / enzymes (at the site of the wound); accept platelets stick together / to the damaged vessel to form a plug(1)
(a) (causing) soluble fibrinogen to be converted to insoluble fibrin; accept fibres / protein threads form(1)
(a) fibrin forms a mesh that traps red blood cells / platelets, forming a clot / scab(1)
(b) prevents (excessive) blood loss(1)
(b) prevents entry of microorganisms / pathogens / bacteria (into the blood); accept prevents infection(1)
Answer: (a) Platelets release chemicals that convert soluble fibrinogen into insoluble fibrin; fibrin threads form a mesh that traps red blood cells to form a clot (b) stops blood loss; stops pathogens entering
Examiner tip: Write 'fibrin forms a mesh that traps red blood cells'. Saying only 'platelets clot the blood' does not explain how a clot forms.

8Grade 6–7Triple only5 marks

A person was given a vaccine on day 0. On day 40 they were exposed to the live pathogen. The graph shows the concentration of antibodies against this pathogen in their blood.

(a) How many times higher is the peak antibody concentration after exposure to the pathogen than the peak after vaccination? [1]

(b) Use the graph to give one other difference between the antibody response after vaccination and the response after exposure to the pathogen. [1]

(c) Explain why the antibody response after exposure to the pathogen is different from the response after vaccination. [3]

(a) 10 (times); accept 100 ÷ 10(1)
(b) antibodies are produced sooner / faster after exposure; accept peak reached after 8 days instead of 14 days; accept antibodies start to rise after about 1 day instead of about 5 days; accept antibody concentration stays high after exposure but falls after vaccination; ignore 'more antibodies' (already covered in (a))(1)
(c) vaccine contains (dead / weakened) pathogen / antigens which stimulate lymphocytes (to produce antibodies)(1)
(c) memory cells are made (after vaccination) and remain in the blood(1)
(c) on exposure, memory cells recognise the pathogen / antigen and (divide rapidly to) produce antibodies sooner, faster and in greater quantity; accept so the pathogen is destroyed before symptoms develop(1)
Answer: (a) 10 times (b) antibodies produced sooner/faster (peak after 8 days, not 14) (c) the vaccine stimulated lymphocytes to make memory cells; on exposure these recognise the pathogen and produce antibodies sooner, faster and in greater quantity
Examiner tip: Name memory cells. Writing 'the body remembers the disease' will not earn the mark.

9Grade 8–9Triple only6 marks

Two children, X and Y, are exposed to the measles virus at the same time. Child X was vaccinated against measles two years earlier. Child Y has never been vaccinated and has never had measles.

Child Y becomes ill with measles but child X does not.

Explain how the immune system responds in each child and why only child Y becomes ill. [6]

vaccine contains dead / weakened / inactive measles virus (or its antigens)(1)
vaccine antigens stimulate lymphocytes to produce antibodies specific to the measles virus(1)
vaccination causes memory cells to be made, which stay in the blood for years(1)
child X: on exposure, memory cells recognise the measles antigens(1)
child X: antibodies are produced sooner, faster and in greater quantity (secondary response)(1)
antibodies bind to the virus / cause clumping / mark it for destruction(1)
phagocytes engulf / ingest the (antibody-coated) viruses and digest them with enzymes(1)
child X: virus is destroyed before it can reproduce enough to cause symptoms(1)
child Y: no memory cells, so lymphocytes that make the right antibody must first be stimulated and multiply(1)
child Y: antibody production is slow / delayed (several days) and smaller at first, so the virus reproduces and damages cells, causing symptoms(1)
child Y: after recovering, will also have memory cells (immune to future infection)(1)

Level	Marks	What it looks like
Level 3	5–6	A detailed, well-linked explanation that compares the two children. It explains how vaccination produces memory cells and how these give child X a faster, larger antibody response that destroys the virus before symptoms develop, contrasting this with the slow first response in child Y. Correct use of terms (lymphocyte, phagocyte, antibody, memory cell).
Level 2	3–4	Explains some of the immune response with some linking, e.g. vaccination leads to memory cells and faster antibody production, but the comparison with child Y is incomplete or some steps are missing or confused.
Level 1	1–2	Simple statements with little linking, e.g. the vaccine makes the child immune, or white blood cells make antibodies / kill the virus.
Level 0	0	No relevant content.

Answer: Vaccine antigens stimulate lymphocytes to make specific antibodies and memory cells. In X, memory cells give a rapid, large antibody response, and phagocytes destroy the virus before symptoms appear. Y has no memory cells, so the first response is slow and small, the virus multiplies and Y becomes ill.
Examiner tip: Top-band answers compare the two children directly using 'sooner, faster, greater quantity' and memory cells, rather than describing vaccination in general.

10Grade 4–54 marks

The table shows blood test results for three patients.

Patient	Red blood cells / millions per mm³	White blood cells / per mm³
P	3.2	7 000
Q	5.0	18 000
R	4.9	6 500
Normal range	4.5 to 5.5	4 000 to 11 000

(a) Which patient is most likely to have a bacterial infection? Give a reason for your answer. [2]

(b) Patient P feels tired and becomes breathless very quickly when walking upstairs. Use the data to explain why. [2]

(a) Q(1)
(a) white blood cell count is above the normal range / higher than 11 000 per mm³; accept higher than the other patients; ignore 'high white blood cells' with no comparison; mark is dependent on Q(1)
(b) red blood cell count is below the normal range / lower than 4.5 million per mm³, so less haemoglobin; accept fewer red blood cells to carry oxygen(1)
(b) so less oxygen is carried to the (muscle) cells, so less aerobic respiration / less energy released; accept more anaerobic respiration (causing breathlessness)(1)
Answer: (a) Q: white blood cell count (18 000) is above the normal range (b) P's red blood cell count is below normal, so less haemoglobin and less oxygen reaches the muscles for respiration, so less energy is released
Examiner tip: Use the normal range in the table: say that the value is above or below it, and quote the number.

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