**Question:** The process by which leukocytes are arranged along the endothelium is called:

A. Leukocyte adhesion
B. Leukocyte migration
C. Leukocyte rolling
D. Leukocyte extravasation

**Core Concept:**

The process of leukocytes (white blood cells) moving from blood vessels into tissues is a crucial part of the immune response and inflammation. This process is usually referred to as leukocyte recruitment or leukocyte migration. It consists of several steps, collectively known as the leukocyte extravasation process. These steps include:

1. **Leukocyte rolling:** Leukocytes adhere to the endothelium, which is the inner lining of blood vessels, through selectins and selectin ligands. This is the initial step, where leukocytes move slowly along the endothelium without detaching.
2. **Leukocyte activation:** Leukocytes become activated, leading to the expression of adhesion molecules, such as integrins, on their surface.
3. **Leukocyte firm adhesion:** Leukocytes attach firmly to the endothelium through integrins and endothelial adhesion molecules.
4. **Leukocyte diapedesis:** Leukocytes cross the endothelium and the basal membrane, entering the tissue space.
5. **Leukocyte extravasation:** Leukocytes reach the site of inflammation or infection and perform their functions.

**Why the Correct Answer is Right:**

The correct answer is D: **Leukocyte extravasation**. This term encompasses the entire process of leukocytes moving from blood vessels into tissues, including the four steps mentioned above.

**Why Other Options are Incorrect:**

A. **Leukocyte adhesion:** While adhesion is a crucial step in the process, it is only a part of leukocyte extravasation and not the entire process.
B. **Leukocyte migration:** Similar to adhesion, migration is a part of the process but does not encompass the entire leukocyte extravasation.
C. **Leukocyte rolling:** This step is essential for the subsequent steps but is not the final outcome of the process.

**Clinical Pearl:**

Understanding leukocyte recruitment and extravasation is essential for medical professionals to comprehend the immune response and inflammation. This process helps in diagnosing and treating various diseases, such as inflammatory disorders, autoimmune diseases, and infections.