Panel 23. Many viruses, like the dengue virus (also see panel 54), enter the cell by endocytosis. Endocytosis is a physiological mechanism to allow molecules present in the extracellular environment to penetrate within the cells. The cell (in green) curves its membrane towards its cytoplasm to form a vesicle that will individualize. The cell thus captures extracellular fluid that contains nutrients, but also certain molecules that can specifically enter the cell via a receptor. For example, the cell allows by such mechanism the entry of transferrin, a protein that regulates the concentration of iron inside the cell.