Primary lymphoid organs are

Options

A. Lymphoid structures formed directly over the lymph vessels
B. Structures formed in the foetus
C. Structures where lymphocytes mature/undergo preprocessing
D. Lymphatic structures where mature lymphocytes reside

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about primary lymphoid organs. We need to identify which of the provided options correctly describes these organs.

2. **Classification of Immune Organs**: Immune organs are classified into two types:
- Primary lymphoid organs (also known as generative organs)
- Secondary lymphoid organs (also known as peripheral organs)

3. **Function of Primary Lymphoid Organs**: Primary lymphoid organs are where lymphocytes (a type of white blood cell) first express their antigen receptors and achieve phenotypic and functional maturity. This is crucial for the immune response.

4. **Identifying Primary Lymphoid Organs**: The primary lymphoid organs include:
- **Bone Marrow**: This is where all lymphocytes originate.
- **Thymus**: This is where T cells mature and gain functional competence.

5. **Evaluating the Options**:
- **Option 1**: "Lymphoid structures formed directly over the lymph vessels" - Incorrect.
- **Option 2**: "Structures formed in the fetus" - Incorrect.
- **Option 3**: "Structures where lymphocytes mature and undergo preprocessing" - Correct.
- **Option 4**: "Lymphatic structures where mature lymphocytes reside" - Incorrect.

6. **Conclusion**: The correct answer is the third option: "Structures where lymphocytes mature and undergo preprocessing."