Fall in WBC number is 

Options

A. leucocytopenia
B. leukemia
C. leucocytosis
D. all the above

Answer

### Step-by-Step Solution:

1. **Understanding WBC**: White blood cells (WBCs) are crucial components of the immune system, responsible for fighting infections and protecting the body against diseases.

2. **Identifying the Condition**: A fall in the number of WBCs is referred to as leucocytopenia. This term specifically denotes a low level of white blood cells in the bloodstream.

3. **Definition of Leucocytopenia**: Leucocytopenia can be defined as a condition where there is an abnormally low count of white blood cells, which can lead to increased susceptibility to infections.

4. **Types of Leucocytopenia**: There are different types of leucopenia based on which specific type of white blood cell is low, such as neutropenia (low neutrophils), lymphocytopenia (low lymphocytes), etc.

5. **Symptoms of Leucocytopenia**: Common symptoms associated with leucocytopenia include:
- High fever
- Chills
- Sweating

6. **Differentiating from Other Conditions**: It's important to differentiate leucocytopenia from other related conditions:
- **Leukemia**: A type of cancer that affects blood-forming tissues, leading to an increase in immature white blood cells.
- **Leukocytosis**: A condition characterized by an increased number of white blood cells, often indicating infection or inflammation.

7. **Conclusion**: Therefore, when there is a fall in WBC number, the correct term to describe this condition is leucocytopenia.