Which one among the following parts of blood carries out the function of body defence?

1. Composition of Blood: Plasma and Formed Elements (basic)

Blood is much more than just a red liquid; in biological terms, it is a specialized fluid connective tissue. Think of it as the body’s primary logistics and security network. It consists of two main parts: a liquid matrix called plasma and various specialized cells known as formed elements that are suspended within it. As we see in Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 91, blood acts as a transport medium for food, oxygen, and waste materials, ensuring that every cell in the body receives what it needs to survive.

The plasma is a straw-colored fluid that makes up about 55% of the blood volume. It is primarily composed of water, but it carries vital substances in dissolved form, including digested food, salts, carbon dioxide (CO₂), and nitrogenous wastes. While plasma handles the bulk of dissolved transport, the formed elements (which make up the remaining 45%) perform specialized physiological duties. These include Red Blood Corpuscles (Erythrocytes) for carrying oxygen, White Blood Cells (Leukocytes) for immune defense, and Platelets for repairing damaged vessels Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 94.

To understand the distinct roles of these components, look at the summary table below:

Sources: Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.91; Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94

2. Red Blood Cells (RBCs) and Gas Transport (basic)

In the vast network of the human body, blood acts as a fluid connective tissue, serving as the primary transport system. While blood contains various components like plasma and platelets, the Red Blood Cells (RBCs), or erythrocytes, are the specialized carriers designed for gas exchange. Every cell in your body needs oxygen (O₂) to generate energy and produces carbon dioxide (CO₂) as a waste product. However, as multicellular organisms grow larger, simple diffusion—the natural movement of molecules from high to low concentration—becomes insufficient. To put this into perspective, if we relied solely on diffusion, it is estimated that it would take three years for a single molecule of oxygen to travel from your lungs to your toes! Science, Class X (NCERT 2025 ed.), Life Processes, p. 91

To overcome this limitation, our body utilizes a specialized respiratory pigment called haemoglobin. Found inside the RBCs, haemoglobin has a remarkably high affinity for oxygen, allowing it to "pick up" O₂ in the lungs where it is abundant and "drop it off" in tissues where it is scarce. The structure of the RBC itself is optimized for this task; it is typically biconcave, providing a large surface area for gas exchange. Interestingly, while O₂ is primarily carried by haemoglobin, carbon dioxide is more soluble in water and is therefore mostly transported in a dissolved form within the blood plasma. Science, Class X (NCERT 2025 ed.), Life Processes, p. 90

It is also important to note that haemoglobin levels are not uniform across all individuals. They can vary based on age, gender, and even species. For instance, adult men typically have higher haemoglobin ranges than adult women, and these levels differ significantly when compared to children or even livestock like cows and buffaloes. Science, Class X (NCERT 2025 ed.), Life Processes, p. 91 This variation reflects the different metabolic demands and physiological needs of different bodies.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.90; Science, Class X (NCERT 2025 ed.), Life Processes, p.91

3. The Human Circulatory System Pathways (basic)

In the human body, the heart acts as a sophisticated double pump, ensuring that oxygen-rich blood and oxygen-poor blood never mix. This mechanism is known as Double Circulation, a vital adaptation for warm-blooded animals like us who require high energy to maintain constant body temperatures Science, Life Processes, p.99. The pathway is divided into two distinct circuits: the Pulmonary Circuit (to the lungs) and the Systemic Circuit (to the rest of the body).

The journey begins when deoxygenated blood from the body enters the right atrium. As the atrium contracts, blood is pushed into the right ventricle, which then pumps it toward the lungs for oxygenation. Once the blood is enriched with oxygen in the lungs, it returns to the left atrium. From here, it moves into the left ventricle, the most muscular chamber of the heart, which pumps the oxygenated blood out to the entire body Science, Life Processes, p.92.

It is important to note the structural design: the ventricles have much thicker muscular walls than the atria because they must exert significant pressure to pump blood over longer distances—specifically the left ventricle, which must reach the furthest extremities of the body Science, Life Processes, p.92.

Sources: Science, Life Processes, p.92; Science, Life Processes, p.99

4. Blood Groups and Rh Incompatibility (intermediate)

To understand blood grouping, we must look at the surface of Red Blood Cells (RBCs). While these cells are primarily responsible for transporting oxygen via haemoglobin Science, Class X (NCERT 2025 ed.), Life Processes, p.91, they also carry specific chemical "identity tags" called antigens. The presence or absence of these antigens determines your blood group. In the ABO system, there are two main antigens: A and B. If you have antigen A, you are group A; if you have both, you are group AB; and if you have neither, you are group O.

Blood compatibility is governed by antibodies found in the plasma. These are part of the body's defense mechanism, which typically identifies and destroys foreign pathogens Science, Class X (NCERT 2025 ed.), Life Processes, p.90. If a person with Group A blood receives Group B blood, their anti-B antibodies will attack the donor cells, causing dangerous clumping (agglutination). This is why blood typing is a critical healthcare activity in any medical camp or hospital setting Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.91.

Beyond ABO, we have the Rh factor (Rhesus factor). Most people are Rh positive (Rh+), meaning they possess the D-antigen. Problems arise with Rh Incompatibility, specifically during pregnancy. If an Rh- mother carries an Rh+ fetus, their blood is usually kept separate by the placenta Science, Class X (NCERT 2025 ed.), How do Organisms Reproduce?, p.124. However, during the first delivery, some fetal Rh+ blood may leak into the mother's bloodstream. The mother's immune system then produces anti-Rh antibodies.

In a subsequent pregnancy with another Rh+ fetus, these antibodies can cross the placenta and destroy the fetus's red blood cells. This condition is known as Erythroblastosis Fetalis (or Hemolytic Disease of the Newborn). It can lead to severe anemia or even fatality for the infant unless medical intervention, such as administering anti-Rh antibodies (RhoGAM) to the mother immediately after the first birth, is performed.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.91; Science, Class X (NCERT 2025 ed.), Life Processes, p.90; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.91; Science, Class X (NCERT 2025 ed.), How do Organisms Reproduce?, p.124

5. The Lymphatic System and Tissue Fluid (intermediate)

6. Platelets and the Coagulation Cascade (intermediate)

Imagine our circulatory system as a high-pressure pumping network. For this system to function efficiently, it must remain leak-proof. If a blood vessel is damaged, the resulting drop in pressure could compromise the delivery of oxygen and nutrients to vital organs. To prevent this, the body utilizes platelets (also known as thrombocytes), which are specialized cell fragments circulating in the plasma specifically to manage repairs Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 91, 94.

The process of stopping a leak, known as hemostasis, occurs through a complex coagulation cascade. When an injury occurs, platelets immediately migrate to the site and adhere to the damaged tissue, forming a temporary "platelet plug." However, for a long-term seal, the body must create a stable clot. This is achieved through a chain reaction of enzymes: prothrombin is converted into the active enzyme thrombin, which then transforms fibrinogen (a soluble protein in plasma) into fibrin. Fibrin molecules join together to form long, tough threads that create a microscopic "mesh" over the wound, trapping blood cells and effectively plugging the leak Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 94.

Understanding this cascade is vital because certain external factors can dangerously accelerate or inhibit it. For instance, the venom of the Russell’s Viper is a highly potent coagulant; it triggers the clotting mechanism so aggressively that it can cause widespread internal blood cell and tissue damage Environment, Shankar IAS Academy, Animal Diversity of India, p.191. In a healthy state, this system is a delicate balance: it must be fast enough to prevent excessive blood loss but controlled enough to avoid unnecessary blockages within the vessels.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.91, 94; Environment, Shankar IAS Academy, Animal Diversity of India, p.191

7. White Blood Cells (WBCs): The Body's Defense (exam-level)

While blood is often viewed simply as a red liquid that circulates through our veins, it is actually a sophisticated fluid connective tissue. Within this fluid, different specialized cells perform distinct roles. While Red Blood Cells (Erythrocytes) focus on transporting oxygen via haemoglobin and Platelets (Thrombocytes) handle the vital task of clotting to prevent blood loss Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 94, the White Blood Cells (WBCs), or Leukocytes, serve as the body's primary defense force.

WBCs function as the "soldiers" of the immune system. Their primary mission is to identify, attack, and destroy harmful pathogens such as bacteria, viruses, and other foreign germs. Unlike other blood cells, some WBCs have the remarkable ability to change shape and squeeze through capillary walls to reach the site of an infection in the tissues. They utilize various strategies to protect us, including phagocytosis (engulfing and digesting invaders) and the production of antibodies that neutralize specific toxins or pathogens.

One of the most critical features of our defense system is its ability to "learn." When the body encounters a specific pathogen for the first time, the initial immune response is often relatively low. However, the WBCs create a biological memory of that encounter. Consequently, if the body is exposed to the same pathogen again, the defense response is much faster and more powerful Science, Class VIII, NCERT (Revised ed 2025), Health: The Ultimate Treasure, p. 45. This "immune memory" is the fundamental principle behind how vaccines protect us from diseases.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94; Science, Class VIII, NCERT (Revised ed 2025), Health: The Ultimate Treasure, p.45

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental components of the circulatory system, this question allows you to apply the concept of functional specialization. In your learning path, we discussed how blood is not just a uniform liquid but a complex fluid connective tissue where different cells perform distinct 'jobs' to maintain homeostasis. When you see the term body defence, your mind should immediately connect to the immune system—the body's internal security force designed to seek and destroy invading pathogens.

To reach the correct answer, use a process of elimination based on the physiological roles you've studied. White blood cells (leukocytes) are the only components listed that are biologically equipped to neutralize bacteria, viruses, and other germs. As emphasized in Science, class X (NCERT 2025 ed.) > Chapter 5: Life Processes, these cells are the primary agents of immunity. Therefore, the correct answer is (B) White blood cells. Think of them as the 'soldiers' of the blood, while the other components serve as the 'logistics' and 'repair' crews.

UPSC often includes options that are vital for life but serve entirely different purposes to test your precision. For instance, Red blood cells and the protein Haemoglobin are traps; their role is strictly respiratory—transporting oxygen from the lungs to the tissues. Similarly, Platelets are specialized for clotting and sealing wounds to prevent blood loss, which is a form of maintenance but not 'defence' against infection. Distinguishing between these specific metabolic roles is the key to cracking Biology-based PYQs.