Which of the statements given below are correct ? 1. A person having blood group ‘A’ can donate blood to persons having blood group ‘A’ and blood group ‘AB 2. A person having blood group ‘AB’ can donate blood to persons having blood groups ‘A1, ‘B’, ‘AB’ or ‘O’. 3. A person with blood group ‘O’ can donate blood to persons having any blood group. 4. A person with blood group ‘O’ can receive blood from the person of any of the blood groups. Select the correct answer using the code given below:

1. Composition of Human Blood (basic)

Welcome to your first step in understanding human genetics! Before we dive into complex DNA sequences or hereditary disorders, we must understand the medium that carries the building blocks of life: Blood. In biology, blood is classified as a fluid connective tissue Science, class X (NCERT 2025 ed.), Life Processes, p.91. Think of it as a sophisticated transport system that constantly circulates to ensure every cell in your body receives nutrients and gets rid of waste.

Blood is not just a simple red liquid; it is a complex mixture of a liquid medium and specialized cells. The liquid portion, known as Plasma, makes up about 55% of blood volume. It is primarily water but carries vital substances like salts, nutrients, and proteins. Plasma is responsible for transporting carbon dioxide and nitrogenous wastes in a dissolved form Science, class X (NCERT 2025 ed.), Life Processes, p.91. Suspended within this plasma are three primary types of cells, each with a distinct "job description" in our physiology.

Component	Primary Function
Red Blood Cells (RBCs)	Contain hemoglobin to carry oxygen to tissues Science, class X (NCERT 2025 ed.), Life Processes, p.91.
White Blood Cells (WBCs)	Act as the body's primary defense system against infections and foreign pathogens.
Platelets	Circulate to clot the blood at points of injury, preventing excessive blood loss and maintaining system pressure Science, class X (NCERT 2025 ed.), Life Processes, p.94.

Understanding this composition is crucial because many genetic disorders—like Hemophilia (a clotting issue) or Sickle Cell Anemia (an RBC issue)—are rooted in malfunctions of these specific components. By ensuring the "leakage" in our blood vessels is minimized through platelets, our body maintains the efficiency of the entire circulatory pumping system Science, class X (NCERT 2025 ed.), Life Processes, p.93.

Sources: Science, class X (NCERT 2025 ed.), Life Processes, p.91; Science, class X (NCERT 2025 ed.), Life Processes, p.93; Science, class X (NCERT 2025 ed.), Life Processes, p.94

2. Antigens and Antibodies: The Recognition System (basic)

To understand human genetics and blood disorders, we must first master the body’s 'ID card' system: Antigens and Antibodies. Think of an antigen as a unique molecular flag or 'marker' found on the surface of cells, particularly our Red Blood Cells (RBCs). These markers are often made of proteins or carbohydrates (Environment, Shankar IAS Academy (10th Ed.), Ecology, p.6). Conversely, antibodies are protective proteins circulating in our plasma—the fluid medium of the blood (Science, Class X, Life Processes, p.91). Their job is to recognize and neutralize 'foreign' flags that do not belong in your body. In the context of the ABO blood group system, your blood type is determined by which antigens are sitting on your RBCs. This is a classic example of biological recognition. If you are Blood Group A, your RBCs display 'A' antigens. Your immune system is smart enough not to attack itself, so your plasma will naturally contain Anti-B antibodies to guard against any 'B' type blood entering your system. If a mismatch occurs, these antibodies will bind to the foreign antigens, causing the blood cells to clump together in a potentially fatal reaction.

Blood Group	Antigens (on RBC)	Antibodies (in Plasma)
Group A	A Antigen	Anti-B
Group B	B Antigen	Anti-A
Group AB	Both A and B	None
Group O	None	Both Anti-A and Anti-B

This recognition system explains why Group O is known as the universal donor. Because Group O RBCs have no A or B 'flags' (antigens) on their surface, they can slip past the 'security guards' (antibodies) of any recipient without being detected. On the other hand, Group AB individuals are universal recipients because their plasma lacks both Anti-A and Anti-B antibodies, meaning they won't attack incoming blood of any ABO type.

Sources: Environment, Shankar IAS Academy (10th Ed.), Ecology, p.6; Science, Class X, Life Processes, p.91

3. The ABO Grouping System (intermediate)

To understand the ABO blood group system, we must first look at what makes up our blood. Blood is a fluid connective tissue consisting of a liquid called plasma and various cells, including Red Blood Cells (RBCs), which carry oxygen throughout the body Science , class X (NCERT 2025 ed.), Life Processes, p.91. The ABO system is defined by the presence or absence of specific proteins called antigens on the surface of these RBCs and corresponding antibodies in the plasma. There are two main antigens, A and B, and two main antibodies, anti-A and anti-B. Your body naturally produces antibodies against the antigens you do not have, which is why matching blood types during transfusion is a matter of life and death.

From a genetic perspective, blood type is an inherited trait. As Mendel's principles suggest, we inherit two versions (alleles) of a gene—one from each parent—which determine our blood group Science , class X (NCERT 2025 ed.), Heredity, p.129. In the ABO system, the alleles for A and B are co-dominant (both express if present), while the allele for O is recessive Science , class X (NCERT 2025 ed.), Heredity, p.133. For instance, if a person has blood group O, they must have inherited the O allele from both parents, as the presence of an A or B allele would otherwise mask it.

The compatibility of these groups is summarized in the table below. It is the interaction between the donor's antigens and the recipient's antibodies that determines if a transfusion is safe:

Blood Group	Antigens (on RBC)	Antibodies (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 only	A, B, AB, O (Universal Recipient)
O	None	Anti-A and Anti-B	Universal Donor	O only

Sources: Science , class X (NCERT 2025 ed.), Life Processes, p.91; Science , class X (NCERT 2025 ed.), Heredity, p.129; Science , class X (NCERT 2025 ed.), Heredity, p.133

4. The Rh Factor and Rh Incompatibility (intermediate)

While the ABO system identifies the primary antigens on our red blood cells, the Rh factor (named after the Rhesus monkey where it was first discovered) is a crucial secondary classification. It refers to the presence or absence of a specific protein called the D antigen. If you have this protein, you are Rh-positive (Rh+); if you lack it, you are Rh-negative (Rh-). Because Rh+ is a dominant genetic trait, a child can inherit Rh+ blood even if only one parent carries the gene. Under normal circumstances, Rh- individuals do not have anti-Rh antibodies in their blood, but they will produce them if they are exposed to Rh+ blood, a process known as sensitization.

The most critical clinical implication of this is Rh incompatibility during pregnancy. This occurs when an Rh- mother carries an Rh+ fetus. During the first pregnancy, the mother's blood and the fetal blood are typically kept separate by the placenta, which serves as the exchange site for nutrients and waste Science, How do Organisms Reproduce?, p.124. However, during delivery, a small amount of the baby's Rh+ blood may enter the mother's bloodstream. The mother’s immune system perceives these Rh antigens as foreign invaders and produces anti-Rh antibodies. While the first baby is usually born safely, the mother is now "sensitized."

In a subsequent pregnancy with another Rh+ fetus, these pre-formed antibodies are small enough to cross the placenta and enter the fetal circulation. They begin to destroy the fetus's red blood cells, leading to a life-threatening condition called Erythroblastosis fetalis (Hemolytic Disease of the Newborn). This can cause severe anemia, brain damage, or even fetal death. Modern medicine prevents this by administering an anti-D injection to Rh- mothers shortly after the first birth, which clears the fetal Rh+ cells before the mother's immune system can react to them.

Sources: Science, How do Organisms Reproduce?, p.124

5. Common Genetic Blood Disorders (intermediate)

To understand blood disorders and compatibility, we must first look at how traits are inherited. In humans, each trait is influenced by both paternal and maternal DNA, meaning every child carries two versions (alleles) of a gene Science, class X (NCERT 2025 ed.), Heredity, p.129. This genetic blueprint determines the ABO blood group system, which is defined by the presence or absence of specific proteins called antigens on the surface of red blood cells and antibodies in the plasma.

The immune system uses antibodies to identify and attack foreign substances. Therefore, safe blood transfusion depends on a simple rule: the donor's antigens must not match the recipient's antibodies. For instance, an individual with Blood Group A has A-antigens and anti-B antibodies; they can safely donate to others with Group A or AB, as these recipients won't attack the A-antigen. Conversely, Group AB individuals are 'universal recipients' because they lack both anti-A and anti-B antibodies, but they can only donate to other AB individuals because their cells carry both types of antigens.

Group O is unique because it lacks both A and B antigens on the cell surface. This makes it the universal donor, as there are no antigens for a recipient's immune system to 'see' and attack. However, Group O individuals have both anti-A and anti-B antibodies in their plasma, meaning their bodies will reject any blood that carries A or B antigens. Consequently, a person with blood group O can only receive blood from another group O donor.

Blood Group	Antigens (on RBC)	Antibodies (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 only	A, B, AB, O
O	None	Anti-A and Anti-B	Universal Donor	O only

Sources: Science, class X (NCERT 2025 ed.), Heredity, p.129

6. Blood Compatibility and Transfusion Logic (exam-level)

To understand blood transfusion, we must look at the red blood cells (RBCs) as more than just oxygen carriers (Science, class X (NCERT 2025 ed.), Life Processes, p.91). Each person’s blood has a specific 'identity' determined by antigens (sugar polymers) located on the surface of the RBCs and antibodies (defense proteins) circulating in the plasma. The ABO Blood Group system is based on two antigens—A and B. If your RBCs have antigen A, you are Group A; if they have both, you are Group AB; and if they have neither, you are Group O.

The golden rule of transfusion is simple but critical: The recipient's antibodies must not attack the donor's antigens. If a person with Group B (who carries anti-A antibodies) receives Group A blood, their antibodies will attack the incoming 'foreign' A-antigens, causing the blood to clump (agglutinate), which can be fatal. This is why blood compatibility is a cornerstone of safe medical practice, alongside the ethical guidelines for organ and tissue donation (Science, class X (NCERT 2025 ed.), Life Processes, p.98).

Blood Group	Antigen (on RBC)	Antibodies (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 only	All (Universal Recipient)
O	None	anti-A and anti-B	All (Universal Donor)	O only

As you can see from the logic above, Group O is the Universal Donor because its cells have no antigens to trigger an attack in any recipient. Conversely, Group AB is the Universal Recipient because its plasma lacks antibodies, meaning it won't attack any incoming A or B antigens. However, Group O individuals are 'trapped' by their own defense system—since their plasma contains both anti-A and anti-B antibodies, they can only receive blood from another Group O donor.

Sources: Science, class X (NCERT 2025 ed.), Life Processes, p.91; Science, class X (NCERT 2025 ed.), Life Processes, p.98

7. Solving the Original PYQ (exam-level)

This question masterfully synthesizes your knowledge of antigens and antibodies. To solve this, you must apply the 'Lock and Key' principle of immunology: a donor's antigen must not match the recipient's antibody, or the blood will clump (agglutinate). Statement 1 is correct because blood group A carries the A-antigen; it can safely enter a recipient of group A (who has no anti-A antibodies) or group AB (who has no antibodies at all). Similarly, Statement 3 is correct because group O lacks both A and B antigens on its surface, allowing it to act as the Universal Donor without being detected by the recipient’s immune system, as noted in MedlinePlus Medical Encyclopedia.

To reach the correct answer, (D) 1 and 3 only, you must navigate the reciprocity traps that UPSC frequently employs. Statement 2 is a classic reversal; while blood group AB is the Universal Recipient (it has no antibodies), it is the most restricted donor because its A and B antigens would be attacked by almost any other group. Statement 4 contains a similar logical flip regarding group O. Although O can give to anyone, it can only receive from other O donors because its plasma contains both anti-A and anti-B antibodies which would immediately attack any incoming A or B antigens. Always pause to distinguish between what is on the cell (antigen) and what is in the plasma (antibody) to avoid these common conceptual pitfalls.