**Question:** When leukocytes are arranged along the endothelium of blood vessels it is called -

A. Leukocyte adhesion
B. Leukocyte extravasation
C. Leukocyte recruitment
D. Leukocyte infiltration

**Correct Answer:** B. Leukocyte extravasation

**Core Concept:**
Leukocytes, also known as white blood cells, are crucial components of the immune system responsible for combating infections and foreign substances within the body. They are typically found circulating within blood vessels, but can also accumulate at sites of inflammation or injury. This process is known as leukocyte migration or leukocyte infiltration, which involves several steps including adhesion, rolling, firm adhesion, and transmigration.

**Why the Correct Answer is Right:**
Leukocyte extravasation refers to the final stage of leukocyte migration, where leukocytes leave the bloodstream and enter the surrounding tissues. This occurs through the process of transmigration, which involves crossing the endothelial cell layer of the blood vessel wall and entering the surrounding tissue. This process is essential for the immune system to effectively respond to infections or tissue damage.

**Why Each Wrong Option is Incorrect:**

A. Leukocyte adhesion: While adhesion is an initial step in the process of leukocyte extravasation, it is not sufficient to describe the entire process. Leukocyte extravasation involves not only adhesion but also rolling, firm adhesion, and transmigration.

B. Leukocyte infiltration: Similar to adhesion, infiltration is also an incorrect term to describe the entire process of leukocyte migration. Leukocyte extravasation is a more comprehensive description that includes adhesion, rolling, firm adhesion, and transmigration steps.

C. Leukocyte recruitment: Recruitment is a broader term that describes the process of attracting leukocytes to the site of inflammation or injury, which includes leukocyte extravasation. Leukocyte extravasation is a more specific subset of recruitment that refers to the actual movement of leukocytes from the bloodstream to the tissue site.

D. Leukocyte infiltration: This term is similar to infiltration and is not specific enough to describe the complete process of leukocyte migration. Leukocyte extravasation is a more accurate term that encompasses adhesion, rolling, firm adhesion, and transmigration steps.

**Clinical Pearl:**
Understanding the steps of leukocyte extravasation is crucial for medical professionals, as it contributes to the immune response against infections and tissue repair. Clinicians should be aware of the process to effectively diagnose, treat, and manage patients with inflammatory conditions or immune disorders.