## **Core Concept**
The normal leucocyte count, also known as the white blood cell (WBC) count, is a crucial parameter in assessing the body's immune response. Leucocytes are an essential part of the immune system, helping to fight infections and diseases. The normal range for the leucocyte count varies slightly between adults and children but generally falls within a specific range.

## **Why the Correct Answer is Right**
The correct answer, . (4,000 to 11,000 cells per microliter), represents the typical range for a normal leucocyte count in adults. This range can slightly vary depending on factors such as age, pregnancy, and physical condition. The leucocyte count includes several types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Each type has a specific role and normal range within the total WBC count.

## **Why Each Wrong Option is Incorrect**
- **Option A:** (less than 4,000 cells/μL) is considered leukopenic, indicating a lower than normal white blood cell count, which can increase the risk of infections.
- **Option B:** (11,000 to 20,000 cells/μL) might be considered mildly elevated but can indicate an inflammatory response or infection.
- **Option D:** (more than 20,000 cells/μL) is significantly elevated and can indicate a severe infection, inflammation, or a hematological disorder.

## **Clinical Pearl / High-Yield Fact**
A normal leucocyte count is crucial for the body's defense against pathogens. An abnormal count, either elevated or decreased, can be an indicator of an underlying condition, such as infection, inflammation, or bone marrow disorders. For example, a significantly elevated WBC count can be a marker for acute infection or sepsis, while a decreased count can signal bone marrow suppression or autoimmune diseases.

## **Correct Answer:** . 4000-11000/μL.