Why O Negative Blood Is Kept Ready for Emergencies
Learn what blood plasma contains and what red cells, white cells and platelets do, how the ABO and Rh systems govern transfusion and pregnancy, and the composition and roles of lymph and the lymphatic system.
What is blood actually made of?
A drop of blood looks like a simple red liquid, but it is a connective tissue packed with cells floating in a watery fluid. It carries oxygen and food, fights infection and seals wounds — and the molecules on its red cells decide whose blood you can safely receive.
This lesson covers plasma and blood corpuscles, the ABO and Rh blood groups, and lymph and the lymphatic system.
This lesson covers plasma and blood corpuscles, the ABO and Rh blood groups, and lymph and the lymphatic system.
What does blood plasma contain, and what do red cells, white cells and platelets do?
Plasma, about 55 per cent of blood, is mostly water with proteins, minerals, nutrients and wastes, while the formed elements — red blood cells, white blood cells and platelets — make up about 45 per cent and carry oxygen, fight infection and help blood clot.
Plasma:
- 90 to 92 per cent water and 6 to 8 per cent proteins
- Fibrinogen for clotting, globulins for defence and albumins for osmotic balance
- Minerals such as sodium, calcium and chloride, with glucose, amino acids and lipids in transit
Red blood cells (erythrocytes):
- The most abundant blood cells, about 5 to 5.5 million per cubic millimetre
- Made in red bone marrow; biconcave and without a nucleus in most mammals
- Carry oxygen using haemoglobin; live about 120 days before being destroyed in the spleen
White blood cells (leucocytes):
- Nucleated, colourless and far fewer, about 6000 to 8000 per cubic millimetre
- Granulocytes — neutrophils, eosinophils and basophils; agranulocytes — lymphocytes and monocytes
- Neutrophils, the most abundant white cells, and monocytes engulf microbes; basophils release histamine and heparin; eosinophils fight infections and are linked with allergy; B and T lymphocytes carry out immune responses
Platelets (thrombocytes):
- Cell fragments from megakaryocytes, about 1.5 to 3.5 lakh per cubic millimetre
- Release factors needed for blood clotting
An everyday example. Doctors watch the platelet count of a dengue patient closely, because a falling count raises the risk of bleeding.
The substance. Serum and plasma are not the same — serum is the fluid left after blood clots, so it lacks the clotting factors that plasma contains.
Plasma:
- 90 to 92 per cent water and 6 to 8 per cent proteins
- Fibrinogen for clotting, globulins for defence and albumins for osmotic balance
- Minerals such as sodium, calcium and chloride, with glucose, amino acids and lipids in transit
Red blood cells (erythrocytes):
- The most abundant blood cells, about 5 to 5.5 million per cubic millimetre
- Made in red bone marrow; biconcave and without a nucleus in most mammals
- Carry oxygen using haemoglobin; live about 120 days before being destroyed in the spleen
White blood cells (leucocytes):
- Nucleated, colourless and far fewer, about 6000 to 8000 per cubic millimetre
- Granulocytes — neutrophils, eosinophils and basophils; agranulocytes — lymphocytes and monocytes
- Neutrophils, the most abundant white cells, and monocytes engulf microbes; basophils release histamine and heparin; eosinophils fight infections and are linked with allergy; B and T lymphocytes carry out immune responses
Platelets (thrombocytes):
- Cell fragments from megakaryocytes, about 1.5 to 3.5 lakh per cubic millimetre
- Release factors needed for blood clotting
An everyday example. Doctors watch the platelet count of a dengue patient closely, because a falling count raises the risk of bleeding.
The substance. Serum and plasma are not the same — serum is the fluid left after blood clots, so it lacks the clotting factors that plasma contains.
How do the ABO and Rh blood groups decide who can receive blood, and why do they matter in pregnancy?
The ABO system classifies blood by antigens A and B on red cells and the matching antibodies in plasma, the Rh system by the presence or absence of the Rh antigen, and a mismatch in either can destroy transfused red cells or harm an Rh-positive foetus carried by an Rh-negative mother.
ABO blood groups:
- Group A — antigen A; anti-B antibodies; can receive A and O
- Group B — antigen B; anti-A antibodies; can receive B and O
- Group AB — antigens A and B; no antibodies; can receive A, B, AB and O — the universal recipient
- Group O — no antigens; anti-A and anti-B antibodies; can receive only O — the universal donor
Rh factor:
- Rh-positive people carry the Rh antigen; Rh-negative people do not
- An Rh-negative person exposed to Rh-positive blood makes anti-Rh antibodies
Rh incompatibility in pregnancy:
- The foetal and maternal blood are kept apart by the placenta, so a first Rh-positive pregnancy is usually unaffected
- During delivery, some foetal blood can enter the mother's circulation, and the mother's body starts making anti-Rh antibodies
- In a later Rh-positive pregnancy these antibodies can cross the placenta and destroy foetal red cells, causing erythroblastosis foetalis, with severe anaemia and jaundice
- It is prevented by giving the mother anti-Rh antibodies soon after the first delivery
An everyday example. Hospital blood banks keep O negative red cells ready for emergencies, because they can be given to a patient of any ABO and Rh type when there is no time to test.
The substance. The universal donor label applies to red cells, not plasma — group O plasma contains anti-A and anti-B antibodies, which is one reason blood banks separate blood into components.
ABO blood groups:
- Group A — antigen A; anti-B antibodies; can receive A and O
- Group B — antigen B; anti-A antibodies; can receive B and O
- Group AB — antigens A and B; no antibodies; can receive A, B, AB and O — the universal recipient
- Group O — no antigens; anti-A and anti-B antibodies; can receive only O — the universal donor
Rh factor:
- Rh-positive people carry the Rh antigen; Rh-negative people do not
- An Rh-negative person exposed to Rh-positive blood makes anti-Rh antibodies
Rh incompatibility in pregnancy:
- The foetal and maternal blood are kept apart by the placenta, so a first Rh-positive pregnancy is usually unaffected
- During delivery, some foetal blood can enter the mother's circulation, and the mother's body starts making anti-Rh antibodies
- In a later Rh-positive pregnancy these antibodies can cross the placenta and destroy foetal red cells, causing erythroblastosis foetalis, with severe anaemia and jaundice
- It is prevented by giving the mother anti-Rh antibodies soon after the first delivery
An everyday example. Hospital blood banks keep O negative red cells ready for emergencies, because they can be given to a patient of any ABO and Rh type when there is no time to test.
The substance. The universal donor label applies to red cells, not plasma — group O plasma contains anti-A and anti-B antibodies, which is one reason blood banks separate blood into components.
What is lymph, and what does the lymphatic system do?
Lymph is the clear fluid that seeps out of blood capillaries into the spaces between cells and is collected by lymph vessels; the lymphatic system returns this fluid and its proteins to the blood, absorbs fats and filters out microbes.
How lymph forms. Pressure at the arterial end of capillaries pushes water and small dissolved substances out into the tissues as tissue fluid. Most of it returns to the blood, and the rest drains into lymph capillaries as lymph.
Composition of lymph:
- Similar to plasma but with fewer proteins
- Rich in lymphocytes; no red blood cells or platelets
- Carries nutrients, hormones and fats absorbed from the intestine
The lymphatic system:
- Lymph capillaries and vessels collect lymph and join into larger ducts that empty into large veins
- Lymph nodes filter lymph and hold lymphocytes that trap and destroy microbes
- Lacteals in the intestinal villi absorb fats
- Lymphoid organs such as the spleen, thymus and tonsils support immunity
Functions:
- Returns tissue fluid and proteins to the blood
- Carries absorbed fats from the small intestine
- Produces and circulates lymphocytes for immunity
An everyday example. Swollen, tender glands in the neck during a throat infection are lymph nodes busy trapping and fighting microbes.
The substance. If lymph cannot drain, tissues swell — in filariasis, worms block lymph vessels and cause the severe swelling of the legs called elephantiasis.
How lymph forms. Pressure at the arterial end of capillaries pushes water and small dissolved substances out into the tissues as tissue fluid. Most of it returns to the blood, and the rest drains into lymph capillaries as lymph.
Composition of lymph:
- Similar to plasma but with fewer proteins
- Rich in lymphocytes; no red blood cells or platelets
- Carries nutrients, hormones and fats absorbed from the intestine
The lymphatic system:
- Lymph capillaries and vessels collect lymph and join into larger ducts that empty into large veins
- Lymph nodes filter lymph and hold lymphocytes that trap and destroy microbes
- Lacteals in the intestinal villi absorb fats
- Lymphoid organs such as the spleen, thymus and tonsils support immunity
Functions:
- Returns tissue fluid and proteins to the blood
- Carries absorbed fats from the small intestine
- Produces and circulates lymphocytes for immunity
An everyday example. Swollen, tender glands in the neck during a throat infection are lymph nodes busy trapping and fighting microbes.
The substance. If lymph cannot drain, tissues swell — in filariasis, worms block lymph vessels and cause the severe swelling of the legs called elephantiasis.
Exam tip
What earns full marks on blood and lymph?
Set out ABO answers group by group — antigen, antibody and the groups it can receive from — so every compatibility question can be read straight off your working.
- Plasma about 55 per cent; formed elements about 45 per cent
- Red cells carry oxygen; neutrophils and monocytes engulf microbes; lymphocytes give immunity; platelets help clotting
- Universal donor: O; universal recipient: AB
- Lymph: like plasma with fewer proteins, and rich in lymphocytes
The trap. Writing that group O has no antibodies. Group O has no antigens but both anti-A and anti-B antibodies; AB has both antigens but no antibodies.
- Plasma about 55 per cent; formed elements about 45 per cent
- Red cells carry oxygen; neutrophils and monocytes engulf microbes; lymphocytes give immunity; platelets help clotting
- Universal donor: O; universal recipient: AB
- Lymph: like plasma with fewer proteins, and rich in lymphocytes
The trap. Writing that group O has no antibodies. Group O has no antigens but both anti-A and anti-B antibodies; AB has both antigens but no antibodies.
Did you know
What is the rare Bombay blood group?
People with the Bombay blood group lack the H antigen, the building block from which the A and B antigens are made. A routine test may label them as group O.
But their plasma carries antibodies against the H antigen, which is present on the red cells of groups A, B, AB and even O. So they can safely receive blood only from another person with the Bombay group.
The group is named after the city now called Mumbai, and blood banks keep lists of rare donors so that such patients can be matched in an emergency.
But their plasma carries antibodies against the H antigen, which is present on the red cells of groups A, B, AB and even O. So they can safely receive blood only from another person with the Bombay group.
The group is named after the city now called Mumbai, and blood banks keep lists of rare donors so that such patients can be matched in an emergency.
Exam relevance
How does NEET test blood composition, blood groups and lymph?
Body Fluids and Circulation is a recurring NEET chapter, and its blood section tests exact facts.
What gets asked. Plasma proteins and their roles, counts and life spans of blood cells, the kinds of white blood cells and their functions, ABO antigens, antibodies and compatibility, Rh incompatibility and erythroblastosis foetalis, and the differences between blood and lymph.
Question types. Mostly statement-based and match-the-column questions, often pairing a blood cell with its function.
Why it matters later. Lymphocytes and lymphoid organs return in Human Health and Disease, and the genetics of ABO groups returns in Principles of Inheritance and Variation as multiple alleles and codominance.
The trap that costs marks. Mixing up granulocytes and agranulocytes — neutrophils, eosinophils and basophils are granulocytes, while lymphocytes and monocytes are agranulocytes.
What gets asked. Plasma proteins and their roles, counts and life spans of blood cells, the kinds of white blood cells and their functions, ABO antigens, antibodies and compatibility, Rh incompatibility and erythroblastosis foetalis, and the differences between blood and lymph.
Question types. Mostly statement-based and match-the-column questions, often pairing a blood cell with its function.
Why it matters later. Lymphocytes and lymphoid organs return in Human Health and Disease, and the genetics of ABO groups returns in Principles of Inheritance and Variation as multiple alleles and codominance.
The trap that costs marks. Mixing up granulocytes and agranulocytes — neutrophils, eosinophils and basophils are granulocytes, while lymphocytes and monocytes are agranulocytes.
Key takeaways
What must you be able to do from this lesson?
- Plasma and corpuscles: plasma with water, proteins and solutes; red cells for oxygen, white cells for defence and platelets for clotting
- Blood groups: ABO antigens and antibodies, the universal donor and recipient, and Rh incompatibility in transfusion and pregnancy
- Lymph: tissue fluid collected by lymph vessels, returning proteins, carrying fats and supporting immunity
A patient with blood group B needs red cells. Which groups could safely donate — and which could not?
- Blood groups: ABO antigens and antibodies, the universal donor and recipient, and Rh incompatibility in transfusion and pregnancy
- Lymph: tissue fluid collected by lymph vessels, returning proteins, carrying fats and supporting immunity
A patient with blood group B needs red cells. Which groups could safely donate — and which could not?