![40. Determine the correctness or otherwise of the following Assertion [a] and Reason [r]. Assertion [a]. B cells secrete antibodies against a virus while cytotoxic T cells kill virus-infected cells. Reason [r]. B cells confer active immunity while cytotoxic T cells confer passive immunity. (A) [a] and [r] are true and [r] is the correct reason for [a] (B) [a] and [r] are true but [r] is not the correct reason for [a] (C) [a] is true but [r] is false (D) [a] false but [r] is true](https://www.letstalkacademy.com/wp-content/uploads/2026/06/1-40.jpg)
40. Determine the correctness or otherwise of the following Assertion [a] and Reason [r].
Assertion [a]. B cells secrete antibodies against a virus while cytotoxic T cells kill virus-infected cells.
Reason [r]. B cells confer active immunity while cytotoxic T cells confer passive immunity.
(A) [a] and [r] are true and [r] is the correct reason for [a]
(B) [a] and [r] are true but [r] is not the correct reason for [a]
(C) [a] is true but [r] is false
(D) [a] false but [r] is true
B Cells and Cytotoxic T Cells in Antiviral Immunity
Introduction
The adaptive immune system protects the human body against viruses, bacteria, fungi, and parasites through two complementary defense mechanisms: humoral immunity and cell-mediated immunity. Humoral immunity is mediated by B lymphocytes that produce antigen-specific antibodies, whereas cell-mediated immunity is mediated primarily by T lymphocytes that eliminate infected cells and regulate immune responses.
During a viral infection, antibodies produced by B cells neutralize viruses before they enter host cells, while cytotoxic T lymphocytes (CD8+ T cells) recognize and destroy virus-infected cells, thereby preventing further viral replication. Together, these two arms of adaptive immunity provide highly effective protection against intracellular pathogens.
Correct Answer
Correct Option: (C) Assertion is true but Reason is false.
Detailed Explanation
The assertion is completely correct. B lymphocytes differentiate into plasma cells that synthesize and secrete antibodies specific for viral antigens. These antibodies bind to viruses, neutralize them, block their attachment to host cells, activate the complement system, and promote phagocytosis through opsonization. Antibody-mediated defense is known as humoral immunity.
Once viruses enter host cells, antibodies can no longer eliminate them effectively because antibodies cannot penetrate living cells. At this stage, cytotoxic T lymphocytes (CD8+ T cells) become the major defensive mechanism. These cells recognize viral peptides presented on MHC class I molecules and destroy infected cells by releasing perforin and granzymes or by activating apoptotic pathways. Consequently, cytotoxic T cells are essential for controlling intracellular viral infections.
The reason, however, is incorrect because it incorrectly states that cytotoxic T cells confer passive immunity. Passive immunity refers to protection obtained by the transfer of preformed antibodies from another individual, such as maternal antibodies transferred through the placenta or antibodies administered therapeutically. Cytotoxic T cells are not transferred as passive immunity in normal physiological immune responses. Instead, both B cells and cytotoxic T cells are components of the body’s own adaptive immune response and therefore contribute to active immunity.
Thus, the assertion is true, but the reason is false.
Explanation of the Assertion
Why the Assertion is Correct
B cells produce virus-specific antibodies that neutralize extracellular viruses, whereas cytotoxic T cells eliminate virus-infected host cells by inducing apoptosis. These complementary immune mechanisms provide effective antiviral protection.
Explanation of the Reason
Why the Reason is Incorrect
The reason incorrectly classifies cytotoxic T cells as mediators of passive immunity. Passive immunity involves receiving ready-made antibodies from another source and does not involve activation of the recipient’s immune cells. Cytotoxic T cells participate in active, cell-mediated immunity, not passive immunity.
Explanation of Each Option
Option (A)
This option is incorrect. Although the assertion is true, the reason is false. Therefore, the reason cannot explain the assertion.
Option (B)
This option is incorrect. The reason itself is false because cytotoxic T cells do not confer passive immunity.
Option (C)
This option is correct. The assertion is true, while the reason is false.
Option (D)
This option is incorrect. The assertion is correct, whereas the reason is incorrect.
Comparison of B Cells and Cytotoxic T Cells
| Feature | B Cells | Cytotoxic T Cells (CD8+) |
|---|---|---|
| Type of Immunity | Humoral Immunity | Cell-Mediated Immunity |
| Main Function | Produce Antibodies | Kill Virus-Infected Cells |
| Target | Extracellular Pathogens | Intracellular Pathogens |
| Recognition Molecule | Native Antigen | MHC Class I-Presented Peptide |
| Effector Mechanism | Neutralization and Opsonization | Perforin and Granzyme Release |
Active and Passive Immunity
| Feature | Active Immunity | Passive Immunity |
|---|---|---|
| Source of Protection | Individual’s Own Immune Response | Preformed Antibodies from Another Source |
| Memory Cells Formed | Yes | No |
| Duration | Long-lasting | Temporary |
| Examples | Natural Infection, Vaccination | Maternal Antibodies, Antiserum Therapy |
Major Cells of Adaptive Immunity
| Cell Type | Main Function |
|---|---|
| B Lymphocytes | Produce antibodies |
| Plasma Cells | Secrete immunoglobulins |
| Helper T Cells (CD4+) | Activate B cells and cytotoxic T cells |
| Cytotoxic T Cells (CD8+) | Destroy infected and abnormal cells |
| Memory B and T Cells | Provide long-term immune memory |
Immune Response Against Viral Infection
| Stage | Immune Response |
|---|---|
| Virus enters the body | Innate immunity is activated |
| Antigen presentation | Activation of helper T cells |
| B-cell activation | Production of virus-specific antibodies |
| CD8 T-cell activation | Recognition of infected cells |
| Effector phase | Neutralization of viruses and killing of infected cells |
| Memory formation | Long-term protective immunity |
Biological Significance
The coordinated action of B cells and cytotoxic T cells allows the immune system to eliminate viruses at multiple stages of infection. Antibodies prevent viruses from infecting cells, whereas cytotoxic T cells eliminate cells that have already become infected. This dual strategy minimizes viral spread, accelerates recovery, and generates immunological memory that protects against future infections. Vaccines exploit this principle by stimulating both humoral and cell-mediated immunity to establish long-lasting protection.
Final Answer
Correct Option: (C) Assertion is true but Reason is false.
The assertion is correct because B cells produce antibodies against viruses, while cytotoxic T cells destroy virus-infected cells. The reason is incorrect because both B cells and cytotoxic T cells contribute to active immunity; passive immunity involves the transfer of preformed antibodies and is not mediated by cytotoxic T cells.
