What is true of leucocytes?

Options

A. These can squeeze out through ( can cross ) the capillary walls
B. These are enucleate
C. Sudden fall in their number indicates cancer
D. These are produced in thymus

Explanation

### Step-by-Step Solution:

1. **Understanding Leukocytes**:
- Leukocytes, also known as white blood cells (WBCs), are a crucial component of the immune system. They are colorless cells that help defend the body against infections and foreign invaders.

2. **Identifying the True Statement**:
- The question asks for a true statement regarding leukocytes. We will evaluate each statement provided.

3. **Evaluating the Statements**:
- **Statement 1**: "Leukocytes can squeeze out through the capillary walls."
- This statement is true. Leukocytes can change shape and use pseudopodia to move out of the bloodstream and into tissues, especially during inflammation. This process is known as diapedesis.

- **Statement 2**: "RBCs are enucleated, the WBCs are enucleated."
- This statement is false. While red blood cells (RBCs) are indeed enucleated (lack a nucleus), leukocytes (WBCs) have a nucleus, which can be multi-lobed in some types.

- **Statement 3**: "A sudden fall in their number will indicate cancer."
- This statement is false. A sudden rise in the number of leukocytes is typically indicative of conditions such as leukemia (blood cancer), not a fall.

- **Statement 4**: "Leukocytes are produced in the thymus."
- This statement is false. While some types of white blood cells (like T-lymphocytes) mature in the thymus, all blood cells, including leukocytes, are primarily produced in the bone marrow.

4. **Conclusion**:
- After evaluating all statements, the only true statement regarding leukocytes is that they can squeeze out through the capillary walls (Statement 1).

### Final Answer:
The true statement about leukocytes is: "Leukocytes can squeeze out through the capillary walls."