BiologyCBSE Class 11 · Body Fluids and Circulation

Blood: Composition, ABO and Rh Blood Groups, and Coagulation

Updated 17 October 2026 · Fact-checked

Blood is a fluid connective tissue made of plasma (55 per cent) and formed elements (45 per cent): RBCs, WBCs and platelets. ABO groups depend on antigens A and B on RBCs and matching antibodies in plasma; group O is the universal donor and AB the universal recipient. An Rh-negative mother carrying an Rh-positive foetus can develop erythroblastosis foetalis. Clotting converts fibrinogen to fibrin through thrombin.

Follow the arrows: O has no antigens so it donates to all, while AB has both antigens so it can receive from all.

Illustration drawn for QuestionBuddy.

Understand Blood Groups and Coagulation of Blood

Composition. Plasma (about 55 per cent) is a straw-coloured viscous fluid: 90-92 per cent water, 6-8 per cent proteins (fibrinogen, globulins, albumins), minerals, glucose, amino acids, lipids; serum is plasma without clotting factors. Formed elements (45 per cent): RBCs (erythrocytes) about 5 to 5.5 million per mm³, no nucleus (in most mammals), contain haemoglobin (12 to 16 g per 100 mL), life span about 120 days, destroyed in the spleen; WBCs (leucocytes) about 6000 to 8000 per mm³ in the order neutrophils (60-65%), lymphocytes (20-25%), monocytes (6-8%), eosinophils (2-3%), basophils (0.5-1%); platelets (thrombocytes) about 1.5 to 3.5 lakh per mm³, for clotting.

ABO blood groups. Based on two surface antigens (A and B) on RBCs; plasma has natural antibodies against the antigens not present.
| Group | Antigen on RBC | Antibody in plasma | Can donate to | Can receive from |
| A | A | anti-B | A, AB | A, O |
| B | B | anti-A | B, AB | B, O |
| AB | A and B | none | AB | A, B, AB, O |
| O | none | anti-A and anti-B | A, B, AB, O | O |
Blood group O is the universal donor and AB the universal recipient. Matching is necessary to avoid agglutination (clumping).

Rh grouping. An antigen similar to one found in the rhesus monkey is called Rh antigen; present in about 80 per cent of humans (Rh positive), absent in the rest (Rh negative). An Rh-negative person exposed to Rh-positive blood forms anti-Rh antibodies. Erythroblastosis foetalis: during pregnancy, if an Rh-negative mother carries an Rh-positive foetus, Rh antigens do not normally mix with the mother's blood during the first pregnancy because the placenta separates the two, but at delivery a small amount of foetal blood can enter the mother's blood; the mother then makes anti-Rh antibodies. In a later pregnancy with an Rh-positive foetus these antibodies cross the placenta and destroy the foetal RBCs, causing anaemia and jaundice, possibly fatal. Prevention: give the mother anti-Rh antibodies immediately after the first delivery.

Coagulation of blood. Prevents blood loss from injury. Platelets and damaged tissues release thromboplastin (clotting factors). Steps: inactive prothrombin → active thrombin (with Ca²⁺) → thrombin converts soluble fibrinogen to insoluble fibrin threads forming a network that traps dead and damaged formed elements, forming the clot (coagulum). Calcium ions are needed for coagulation.

Lymph. Colourless fluid with lymphocytes; carries nutrients and hormones, fat absorbed from the lacteals; returns tissue fluid to blood.

Flow chart: injury → platelets/tissue release thromboplastin → prothrombin → thrombin (Ca²⁺) → fibrinogen → fibrin → clot.

Blood Groups and Coagulation of Blood: key facts

Blood: Plasma 55% | formed elements 45%; RBC, WBC, platelets
ABO: A: antigen A, anti-B
B: antigen B, anti-A
AB: A and B, none
O: none, anti-A and anti-B; Antigens and antibodies
Clotting: $P r o t h r o mbin - > t h r o mbin (C a 2 +) ∣ f ib r in o g e n - > f ib r in$; With thromboplastin
Rh: Rh- mother + Rh+ foetus | anti-Rh after first delivery; Erythroblastosis foetalis

How to answer Blood Groups and Coagulation of Blood questions

Questions give a group and ask for compatible donors, or describe a Rh situation.

1Write the antigens and antibodies of the recipient.
2A donor is compatible if its RBC antigens do not meet matching antibodies in the recipient plasma.
3For Rh, check the mother and the foetus and the pregnancy number.
4For coagulation, write the chain with Ca2+.
5Quote percentages and counts accurately.

Blood Groups and Coagulation of Blood in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Accident and transfusion

Suppose an injured person with blood group B needs an urgent transfusion and only O and A units are available.

Which unit can be given, and why?

Try it first, then show the solutionSolution

Group B has antigen B and anti-A antibodies.
Group O cells have no antigen, so they cannot be clumped; group A cells would meet anti-A and clump.
So the O unit can be given.

Answer: Give group O blood.

Memory hook: O has nothing for antibodies to attack.

Tricks to remember Blood Groups and Coagulation of Blood

Short memory aids. Say each one aloud once, then use it on a question the same day.

O gives, AB takes
O universal donor; AB universal recipient.
Use it when: ABO
Anti = opposite
Group A carries anti-B.
Use it when: Antibodies
Pro-Thromb-in
Prothrombin to thrombin.
Use it when: Clotting

Blood Groups and Coagulation of Blood: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Universal donor · 2 marks · Very short answerBoard level

Why is O the universal donor and AB the universal recipient?

Try it first, then show the solutionSolution

O has no antigen on RBCs, so recipients' antibodies find nothing to agglutinate (1 mark).
AB has no antibodies in plasma, so it can accept any RBC type (1 mark).

Answer: O: no antigen; AB: no antibodies.

Example 2: Clotting · 3 marks · Short answerBoard level

Describe the steps in blood clotting.

Try it first, then show the solutionSolution

Injured tissues and platelets release thromboplastin (1 mark).
Prothrombin to thrombin in the presence of Ca2+ (1 mark).
Thrombin converts fibrinogen to fibrin threads that form the clot with trapped cells (1 mark).

Answer: Thromboplastin, thrombin, fibrin.

Example 3: Transfusion table · 3 marks · Short answerHarder board level

A patient of group A needs blood. Which groups can donate, and what happens if group B blood is given? Explain with antigens and antibodies.

Try it first, then show the solutionSolution

Donors: A and O (1 mark).
Group A plasma has anti-B antibodies (1 mark).
Group B RBCs carry antigen B, so agglutination occurs (1 mark).

Answer: A and O; B causes clumping.

Example 4: Erythroblastosis · 4 marks · Short answerHarder board level

Explain erythroblastosis foetalis step by step, and how it is prevented.

Try it first, then show the solutionSolution

Mother Rh-negative, foetus Rh-positive (1 mark).
At first delivery foetal blood enters maternal blood and she makes anti-Rh antibodies (1 mark).
In the next Rh-positive pregnancy, antibodies cross the placenta and destroy foetal RBCs (1 mark).
Prevention: give anti-Rh antibodies to the mother right after the first delivery (1 mark).

Answer: Sensitisation at first delivery harms the second child.

Example 5: Blood counts · 4 marks · Short answerHOTS / competency

A sample shows 6500 WBC per mm3. If neutrophils are 62 per cent and lymphocytes 24 per cent, find the number of each per mm3. Why are the RBC counts much higher than WBC? What fraction remains for others?

Try it first, then show the solutionSolution

Neutrophils = 0.62 x 6500 = 4030 per mm3 (1 mark).
Lymphocytes = 0.24 x 6500 = 1560 per mm3 (1 mark).
Others = 100 - 62 - 24 = 14 per cent = 910 per mm3 (1 mark).
RBCs (about 5 million) carry oxygen in bulk, hence outnumber WBCs (1 mark).

Answer: 4030 neutrophils; 1560 lymphocytes; 910 others.

Where students lose marks in Blood Groups and Coagulation of Blood

The slips examiners see most often, and what to write instead.

Mistake: Antigen in plasma
Location mixed.
Wrong working
Antigens are in plasma.
Right working
Antigens are on RBCs; antibodies in plasma.
Do this: Antigen on cell.
Usually costs: 1 mark.
Mistake: Group O has anti-none
O antibodies wrong.
Wrong working
O has no antibodies.
Right working
O has anti-A and anti-B.
Do this: No antigen, both antibodies.
Usually costs: 1 mark.
Mistake: Rh trouble in first child
Pregnancy order.
Wrong working
First child is always affected.
Right working
Usually the later Rh-positive child is affected.
Do this: Sensitisation at delivery.
Usually costs: 1 mark.

Exam tips

Draw the ABO table.
Give percentages of WBCs.
Mention Ca2+ in clotting.
Explain Rh using the term anti-Rh.

Practice questions on Blood Groups and Coagulation of Blood

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
A person with blood group AB has
(a) anti-A only (b) anti-B only (c) both antibodies (d) no antibodies
Hint
Universal recipient.
Answer
(d).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A second Rh-positive child of an Rh-negative mother is at risk.
Reason (R): Anti-Rh antibodies from the first pregnancy can cross the placenta.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Sensitisation.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
Name the plasma protein that forms the clot and the enzyme that converts it.
Hint
Fibrin.
Answer
Fibrinogen forms fibrin; thrombin converts it.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a lab finds RBC agglutination when serum from a person is mixed with group B cells but not with group A cells.
(a) State the antibodies in the serum. (1)
(b) Identify the group. (1)
(c) Which groups could receive this person's blood? (2)
Hint
Anti-B but no anti-A.
Answer
(a) Anti-B (no anti-A). (b) Group A (group O would also have anti-A and would agglutinate A cells). (c) A and AB.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a hospital stores blood without calcium chelator and the sample clots in the tube. Explain which factors are involved and how citrate prevents clotting.
Hint
Ca2+ needed.
Answer
Clotting needs thromboplastin, prothrombin, Ca2+ and fibrinogen. Adding citrate binds Ca2+ so thrombin cannot form, and clotting is prevented for storage.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Life span of RBC?Show answer
About 120 days.
2.Percentage plasma?Show answer
55%.
3.Platelet count?Show answer
1.5 to 3.5 lakh per mm3.
4.Universal donor?Show answer
O.
5.Clot protein?Show answer
Fibrin.
6.Most abundant WBC?Show answer
Neutrophils.

Blood Groups and Coagulation of Blood: frequently asked questions

What is plasma?

The fluid part of blood.

What is agglutination?

Clumping of RBCs.

What is serum?

Plasma without clotting factors.

What is Rh factor?

An antigen on RBCs of Rh-positive people.

Why is Ca2+ needed in clotting?

To convert prothrombin to thrombin.