This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Transmigration of leukocytes occurs mainly through which of the following? 
Select and explain. 
A. Aeriole, B. Venule, C. Capillary, D. All of the above
B: Venule
Answer: B: Venule 
Explanation: Transmigration of leukocytes occurs mainly in post-capillary venules. Chemokines act on the adherent leukocytes and stimulate the cells to migrate through interendothelial spaces toward the chemical concentration gradient, that is, toward the site of injury or infection where the chemokines are being produced. Ref: Robbins 8th edition Chapter 2.