Final answer:
The molecule upregulated on the surface of effector T cells to facilitate their homing to inflamed tissue is VLA-4 (option a). Effector T cells use VLA-4 to bind to VCAM-1 on inflamed tissue endothelial cells, allowing the T cells to enter the site of inflammation.
Step-by-step explanation:
The question asks which molecule is upregulated on the surface of effector T cells to facilitate their homing to inflamed tissue. The correct answer is VLA-4, which stands for Very Late Antigen-4.
Effector T cells, which are activated T cells, use VLA-4 to bind to VCAM-1 (vascular cell adhesion molecule-1) on the endothelial cells of blood vessels in inflamed tissues. This adhesion is crucial for the T cells to exit the bloodstream and enter the tissue where they can exert their effects against pathogens.
For clarity, the options given were: a. VLA-4, b. L-Selectin, c. CD28, d. VCAM. Although L-Selectin and CD28 play roles in T cell activation and trafficking, they are not the main facilitators for effector T cell homing to inflamed tissue. VCAM is not upregulated on T cells but rather on endothelial cells.