Viruses find various ways to take advantage of and trick cells. Many of the forms of viral entry in the graphic above take advantage of a cell’s normal processes. Cells consume nutrients and bond and interact with other cells, and viruses can find their way into cells during these processes.
Fusion, as shown in the graphic, is the process where a virus can fuse with a cell’s plasma membrane. Once the virus is attached to the cell via the cell’s receptors, the envelope on the outside of the virus can fuse with the cell's plasma membrane, and release the contents of the virus into the cell, leaving the envelope on the outside.
Cell-to-Cell fusion is a normal process that cells undergo without a virus present. This process occurs during the development of embryos, muscles, bones, and during other processes of the human body. If one cell contains a virus and fuses with another cell, the virus will spread to the previously healthy cell.
Pinocytosis is also a normal process for cells to undergo, but viruses can take advantage of it. Pinocytosis can be compared to drinking because cells take in liquid nutrients. The cell membrane allows a hole to form, letting liquid nutrients into it. It then pinches off nutrients, allowing liquid matter to enter the cell.
Phagocytosis is similar to pinocytosis in that it is another way for the cell to get nutrients and engulf other extracellular matter. Dead cells, microorganisms, other particulate matter, and viruses can enter the cell via phagocytosis. Unlike pinocytosis, phagocytosis does not pinch off matter, but instead uses more energy in order to surround the extracellular matter.
Clathrin is a protein used to coat vesicles. Clathrin mediated endocytosis is very similar to other types of endocytosis, but with a different method of transport. An indent is made on the outside of the cell, allowing extracellular material to enter this indent. The cell then closes around the extracellular material, and clathrin coats the outside of the vesicle.
Caveolae are small “caves” composed of lipids. Caveolae are used to perform potocytosis, which brings matter of various sizes from outside the cell into the cytosol. Caveolae also allow the cell-signaling to occur. Lastly, caveolae are needed for transcytosis, a process where macromolecules can pass completely through a cell. These macromolecules enter the cell, pass through, and are sent out of the other side. Viruses can enter the cell through one of the various types of caveolae mediated endocytosis.