Which of the following parts of blood carry out the function of body defence ?

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

Blood is much more than just a red liquid; it is a specialized fluid connective tissue that acts as the body's primary transport and defense system. To understand its composition, think of it in two distinct parts: a liquid medium called plasma and various solid components known as formed elements (cells and cell fragments) that are suspended within it Science, Class X, Life Processes, p. 91.

Plasma makes up about 55% of your blood volume. It is a straw-colored liquid consisting mostly of water, but it serves a critical role in transporting dissolved substances. While oxygen relies on specialized cells, plasma is the primary carrier for food (nutrients), carbon dioxide (CO₂), and nitrogenous wastes like urea. It also carries salts and hormones, ensuring every part of the body receives what it needs and can discard what it doesn't Science, Class X, Life Processes, p. 91.

The remaining 45% consists of the formed elements, each with a specialized role:

• Red Blood Corpuscles (Erythrocytes): These are packed with hemoglobin, a protein that binds to oxygen. Their sole mission is to carry oxygen from your lungs to every tissue in your body Science, Class X, Life Processes, p. 91.
• White Blood Cells (Leukocytes): These are the soldiers of your immune system. They defend the body against foreign invaders like bacteria and viruses. Some produce antibodies, while others directly attack infected cells.
• Platelets (Thrombocytes): These are tiny cell fragments that act as a repair crew. If a blood vessel is damaged, platelets rush to the site to facilitate clotting, preventing excessive blood loss and sealing the system Science, Class X, Life Processes, p. 91.

Sources: Science, Class X, Life Processes, p.91

2. Red Blood Cells (Erythrocytes) and Oxygen Transport (basic)

In large multicellular organisms like us, simply letting oxygen soak into our skin through diffusion isn't enough to reach every cell. To solve this, our blood acts as a fluid connective tissue, a specialized transport system that ensures every tissue receives vital nutrients and gases Science, Class X, Chapter 5: Life Processes, p.91. While blood consists of a liquid medium called plasma, it is the specialized cells suspended within it that perform the heavy lifting of gas transport.

Red Blood Cells (RBCs), or erythrocytes, are the body's primary oxygen couriers. Their efficiency comes from a remarkable iron-rich protein called haemoglobin. Haemoglobin is often called a respiratory pigment because it has a very high affinity for oxygen; it grabs O₂ in the lungs and holds onto it until it reaches tissues that are oxygen-deficient Science, Class X, Chapter 5: Life Processes, p.90. Interestingly, while RBCs handle oxygen, most of the carbon dioxide (CO₂) is carried in the dissolved form within the plasma because CO₂ is more soluble in water than oxygen is Science, Class X, Chapter 5: Life Processes, p.90.

To understand why RBCs are so specialized, look at how they differ from the rest of the blood components in their transport duties:

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.90; Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.91

3. Human Circulatory System: Heart and Vessels (intermediate)

Welcome back! Now that we understand blood as our transport medium, let’s look at the machinery that moves it. Think of the Human Circulatory System as a sophisticated plumbing network where the heart acts as a central pump and the vessels serve as the distribution pipes. This system is designed for maximum efficiency through a process known as double circulation.

The heart is a four-chambered muscular organ. The separation of the right and left sides is crucial because it prevents oxygen-rich blood from mixing with carbon dioxide-rich blood. This separation allows for a highly efficient supply of oxygen to the body, which is essential for warm-blooded animals like us to maintain a constant body temperature Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.99. In double circulation, blood travels through the heart twice: once to the lungs to get oxygenated (pulmonary circuit) and once to the rest of the body (systemic circuit).

The "pipes" or blood vessels are specialized based on their function. Arteries carry blood away from the heart; because this blood is under high pressure, arteries have thick, elastic walls. In contrast, veins collect blood from different organs and bring it back to the heart. Since the pressure is lower here, their walls are thinner, but they contain valves to ensure blood only flows in one direction. Connecting these are capillaries, the smallest vessels, with walls only one-cell thick to allow for the exchange of materials between blood and cells Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.93.

Finally, we have the lymphatic system. Some plasma, proteins, and cells escape through capillary pores into tissue spaces, forming lymph or tissue fluid. This colorless fluid carries digested fats and drains excess fluid back into the blood, acting as a secondary drainage system Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94.

Sources: Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.93; Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94; Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.99

4. Blood Groups and Rh Factor Genetics (intermediate)

Blood groups are determined by the presence or absence of specific antigens (proteins) on the surface of Red Blood Cells (Erythrocytes) and antibodies in the plasma. While the blood itself is a fluid connective tissue Science, class X (NCERT 2025 ed.), Life Processes, p.91, these surface markers are what make our blood unique. The most critical system is the ABO system, which is governed by a single gene (the I gene) that exists in three versions, or alleles: Iᴬ, Iᴮ, and i.

The inheritance of blood groups is a classic example of Co-dominance and Multiple Alleles. In simple Mendelian genetics, one trait usually hides another (dominance). However, in blood groups, Iᴬ and Iᴮ are both dominant over the recessive i allele, but they are equally dominant to each other. If an individual inherits both Iᴬ and Iᴮ, they express both antigens, resulting in Blood Group AB Science, class X (NCERT 2025 ed.), Heredity, p.133. This is why a person with Blood Group O must have two recessive alleles (ii), as any presence of Iᴬ or Iᴮ would mask it.

Beyond ABO, we have the Rh Factor (Rhesus factor), named after the Rhesus monkeys where it was first discovered. This is a protein found on the RBC surface. If you have it, you are Rh Positive (Rh+); if not, you are Rh Negative (Rh-). Genetically, Rh+ is a dominant trait. Understanding this is vital for blood transfusions and pregnancy, as an Rh- mother carrying an Rh+ fetus may develop antibodies that could affect future pregnancies—a condition known as erythroblastosis fetalis.

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

5. The Lymphatic System and Tissue Fluid (intermediate)

While we often think of the circulatory system as a closed loop of blood vessels, it is actually a bit "leaky." As blood flows through the thin-walled capillaries under pressure, some amount of plasma, proteins, and white blood cells escape through tiny pores into the spaces between cells. This escaped fluid is known as tissue fluid or interstitial fluid. It serves as the immediate environment for our cells, allowing for the exchange of nutrients and gases Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94.

If this fluid were left to accumulate, our tissues would swell dangerously. This is where the lymphatic system steps in as a vital drainage and filtration network. The tissue fluid enters specialized, blind-ended tubes called lymphatic capillaries. Once inside these tubes, the fluid is called lymph. Unlike the circulatory system which is a continuous circle, the lymphatic system is a one-way street, carrying fluid from the tissues back toward the heart, where it eventually rejoins the blood through large veins Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94.

Lymph is essentially similar to blood plasma but with two key differences: it is colorless (because red blood cells are too large to escape the capillaries) and it contains significantly less protein. Beyond fluid balance, the lymphatic system has two other critical roles. First, it acts as a "fat highway"; while most nutrients enter the blood directly, digested and absorbed fats from the intestine are too bulky and must be transported via specialized lymphatic vessels called lacteals. Second, lymph passes through lymph nodes, which act as security checkpoints filled with white blood cells to trap and destroy pathogens Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94.

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

6. Platelets (Thrombocytes) and Blood Coagulation (intermediate)

In our previous discussions, we explored how blood acts as a fluid connective tissue, transporting oxygen via red blood cells and defending the body through white blood cells. However, for this "pumping system" to function efficiently, it must remain a closed circuit. Imagine a puncture in a high-pressure water pipe; the leak would cause the entire system to lose pressure. In our bodies, Platelets (also known as Thrombocytes) serve as the essential maintenance crew that plugs these leaks to prevent excessive blood loss Science, Class X (NCERT 2025 ed.), Life Processes, p.94.

Platelets are not actually whole cells but tiny, disc-shaped cell fragments produced in the bone marrow. When a blood vessel is injured, platelets immediately rush to the site and adhere to the damaged surface. This triggers a complex chemical relay known as the coagulation cascade. The ultimate goal of this process is to convert a soluble protein in the plasma, called fibrinogen, into insoluble threads of fibrin. These fibrin threads form a microscopic net that traps blood cells, creating a stable clot (thrombus) that seals the wound.

Two critical "silent partners" in this process are Calcium ions (Ca²⁺) and Vitamin K. While Calcium is well-known for bone health and plant growth Environment, Shankar IAS Academy (10th ed.), Agriculture, p.363, it is also a vital catalyst in the blood-clotting sequence. Without sufficient Calcium or Vitamin K, the body cannot produce the necessary enzymes to form a clot, which can lead to dangerous internal or external bleeding. Conversely, some substances, like the hemotoxic venom of the Russell's Viper, can cause rapid, pathological clotting that destroys tissues Environment, Shankar IAS Academy (10th ed.), Animal Diversity of India, p.191.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.94; Environment, Shankar IAS Academy (10th ed.), Agriculture, p.363; Environment, Shankar IAS Academy (10th ed.), Animal Diversity of India, p.191

7. White Blood Cells (Leukocytes): The Immune Army (exam-level)

8. Solving the Original PYQ (exam-level)

Having explored the composition of blood as a fluid connective tissue, you can now see how its specialized components work in harmony to maintain homeostasis. This question tests your ability to map specific cellular functions to their respective components. While plasma acts as the medium, the cellular elements—erythrocytes, leukocytes, and thrombocytes—each carry out distinct physiological roles. To identify the "defense" component, you must recall the role of the immune system's primary soldiers.

To arrive at the correct answer, think about which cell type is equipped with mechanisms like phagocytosis and antibody production. The White blood cells (WBCs), or leukocytes, are the only components dedicated to identifying and neutralizing foreign invaders like bacteria and viruses. As noted in Science, class X (NCERT 2025 ed.), these cells operate through complex pathways, such as lymphocytes generating antibodies or T cells directly destroying infected host cells. Therefore, (B) White blood cells is the correct choice for body defense.

UPSC often uses closely related terms to test your precision. Red blood cells (RBCs) and haemoglobins (Option A and D) are distractors focused on respiration; RBCs are the vehicles, while haemoglobin is the specific protein that binds oxygen. Platelets (Option C), or thrombocytes, are specialized for hemostasis (clotting) rather than immunity. Don't fall for the trap of confusing "maintenance" (clotting) with "defense" (immune response). By isolating the function of infection-fighting, WBCs stand out as the clear answer.