Q.19: B Cells and T Cells Immunity Assertion-Reason

Question Statement

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 secrete antibodies and cytotoxic T cells kill virus-infected cells, forming key parts of adaptive immunity. The assertion correctly describes their roles, but the reason misstates their immunity types.

Assertion Analysis

Assertion [a] states that B cells secrete antibodies against viruses while cytotoxic T cells (CD8+ T cells) kill virus-infected cells. B cells differentiate into plasma cells that produce virus-specific antibodies to neutralize pathogens. Cytotoxic T cells recognize viral antigens on MHC class I molecules and induce apoptosis in infected cells via perforin and granzymes.

Reason Analysis

Reason [r] claims B cells confer active immunity while cytotoxic T cells confer passive immunity. Both B cells and cytotoxic T cells contribute to active immunity, where the host’s immune system generates its own response, including memory cells for long-term protection. Passive immunity involves pre-formed antibodies from external sources like maternal transfer, not endogenous cell responses.

Option Evaluation

• (A): Both true, [r] explains [a]. Incorrect—[r] is false.

• (B): Both true, [r] unrelated to [a]. Incorrect—[r] is false.

• (C): [a] true, [r] false. Correct—matches the analysis.

• (D): [a] false, [r] true. Incorrect—[a] is true, [r] false.

Correct answer: (C)

B cells and T cells immunity assertion-reason questions test core concepts in adaptive immunity for CSIR NET Life Sciences. This analysis breaks down the roles of B cells secreting antibodies against viruses and cytotoxic T cells eliminating infected cells, clarifying active versus passive immunity misconceptions.

Key Roles in Viral Defense

B cells produce antibodies that bind viral antigens, preventing entry into host cells and marking them for destruction. Cytotoxic T cells directly target and lyse virus-infected cells through granule exocytosis.

Active vs Passive Immunity

Active immunity arises from the body’s own B and T cell responses, generating memory for lasting protection. Passive immunity provides short-term antibody transfer without memory cell formation—neither B cells nor cytotoxic T cells confer this.