Final answer:
Selectins are the cell adhesion molecules that bind to leukocytes to slow their movement during leukocyte extravasation, a critical part of the immune response. answer A is correct.
Step-by-step explanation:
Selectins are the cell adhesion molecules that bind to leukocytes and slow their movement during leukocyte extravasation. This process is crucial for the immune response, allowing white blood cells to exit the bloodstream and enter tissues where they can fight infections. During leukocyte extravasation, selectins are first to interact with the leukocytes, causing them to roll along the vascular endothelium.
This rolling action slows the cells down and allows them to be activated by chemokines. Upon activation, leukocytes express integrins, which bind strongly to ICAMs (intercellular adhesion molecules) and VCAMs (vascular cell adhesion molecules) on the endothelial cells, stopping the leukocytes and allowing them to squeeze through the blood vessel wall into the tissue.