The site where a B - lymphocytes mature in aves

Options

A. Bone marrow
B. Bursa of fabricius
C. Spleen
D. Lymph nodes

Answer

### Step-by-Step Solution:

1. **Understanding B-lymphocytes**: B-lymphocytes are a type of white blood cell that plays a crucial role in the immune response by producing antibodies to fight off antigens.

2. **B-lymphocyte Maturation in Mammals**: In mammals, including humans, B-lymphocytes mature in the bone marrow. This is a well-established fact in immunology.

3. **B-lymphocyte Maturation in Aves (Birds)**: The maturation of B-lymphocytes in birds (aves) is different from that in mammals. Research has shown that B-lymphocytes mature in a specific lymphoid organ.

4. **Identifying the Organ**: The organ where B-lymphocytes mature in birds is known as the Bursa of Fabricius. This organ is unique to birds and plays a critical role in their immune system.

5. **Conclusion**: Based on the information provided, the correct answer to the question regarding where B-lymphocytes mature in aves is the Bursa of Fabricius.

### Final Answer:
The site where B-lymphocytes mature in aves is the Bursa of Fabricius (Option B).

---