The primary lymphoid organs.

Options

A. Are bone marrow and thymus
B. Differentiate immature lymphocytes into antigen sensitive lymphocytes
C. Provide micro-environments for the development and maturation of T-lymphocytes
D. All of the above

Answer

### Step-by-Step Solution:

1. **Definition of Primary Lymphoid Organs**:
- Primary lymphoid organs are the sites where lymphocytes (a type of white blood cell) are produced and mature. The two main primary lymphoid organs in humans are the bone marrow and the thymus.

2. **Role of Bone Marrow**:
- Bone marrow is responsible for the production of immature lymphocytes. It is the site where B lymphocytes (B cells) originate and mature.

3. **Role of Thymus**:
- The thymus is where T lymphocytes (T cells) mature. Immature T cells migrate from the bone marrow to the thymus, where they undergo maturation and differentiation.

4. **Functionality of Primary Lymphoid Organs**:
- These organs provide a micro-environment that is essential for the development and maturation of lymphocytes. This includes the interaction with various signaling molecules and other cells that facilitate the maturation process.

5. **Summary of Functions**:
- The primary lymphoid organs (bone marrow and thymus) create lymphocytes, differentiate immature lymphocytes into antigen-sensitive lymphocytes, and provide the necessary environment for the development and maturation of T lymphocytes.

6. **Conclusion**:
- Therefore, the correct answer regarding the primary lymphoid organs encompasses all the statements provided, confirming that option 4 is the right choice.