Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.

Assertion A: Birds bursectomized on 18--20 days synthesize IgM only.

Reason R: Competence for IgM production is acquired early on day 14 of embryonation.

In light of the above statements, choose the correct answer from the options given below.

• A. Both A and R are true and R is the correct explanation of A.
• B. Both A and R are true but R is NOT the correct explanation of A.
• C. A is true but R is false.
• D. A is false but R is true.

Hint:

Remember: Day 14 = IgM competence. Bursectomy after this stage allows IgM production, whereas IgG and IgA responses remain deficient due to impaired B-cell maturation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The bursa of Fabricius is the primary lymphoid organ responsible for B-cell maturation in birds.
Bursectomy performed late in embryonic development affects the production of different classes of immunoglobulins.
Detailed Explanation:
Assertion A:
Birds bursectomized on embryonic days 18--20 retain the ability to synthesize IgM because B cells acquire competence for IgM production before hatching.
However, IgG and IgA production is markedly impaired due to the absence of normal bursal maturation.
Therefore, Assertion A is true.
Reason R:
The ability to produce IgM develops around the 14th day of embryonation.
Thus, even after bursectomy at 18--20 days, previously committed B cells can still synthesize IgM.
Hence, Reason R is true.

Step 2: Relationship between Assertion and Reason:
The early acquisition of IgM-producing competence directly explains why late embryonic bursectomy results in the synthesis of IgM only.
Therefore, the Reason correctly explains the Assertion.

Step 3: Final Answer:
Both Assertion A and Reason R are true, and R is the correct explanation of A.
\[ \boxed{(A) Both A and R are true and R is the correct explanation of A. \]