Part: 2 of 3
The body’s lines of defense
The first line of defense
Normally (that is: when we are not wounded), viruses can only enter the body through one of the mucous membranes. Normally, this will be a membrane of the nose, mouth, or throat.
A virus, being a passive construction, cannot actively swim through the mucus in the direction of a cell. Instead, it just sits in the mucus, waiting until chance brings it in contact with a cell.
The body contains intrusion-detecting cells and enzymes that are also present in the mucous linings. If these encounter a virus, they will destroy it. This prevents the virus from entering the body at all.
The second line of defense
A virus that is not destroyed by the first line of defense may eventually reach one of the cells of the mucous membrane. It has to infect these cells in order to enter the body. The mucus cells are highly resilient to virus infection. If infection does occur, they quickly excrete alarm proteins that alert the body to the infection.
(Note: I’m not an expert in this field. Therefore, the above description may be overly simplistic, and it may even be a little off. If you have more knowledge about this second line of defense than I have, please share your knowledge, so I can improve this part of the text.)
The third line of defense
Once the virus makes it through the layer of cells that line the mucous membrane, it is inside the body. The body now depends on its third line of defense to fight the virus. This third line of defense attacks the virus in two ways.
The first possibility is that the body attacks the virus itself. This can only be done with virus particles that have not yet infected a cell. These virus particles are recognised by the proteins in their envelope, which are not human proteins. The virus particles are marked by special markers that have two sides: one side chemically binds to the virus protein, the other side signals immune cells to “kill me”. The immune cells then absorb the markers and the viruses to which these markers are attached into little intracellular bags called lysosomes. The cell then fills the lysosome with destructive enzymes. This kills the virus, reducing it to recyclable amino acids and lipides.
The second possibility is that the body attacks the cells that are already infected by the virus. These cells are doomed anyway, so they may as well be killed immediately. Cells that are infected by a virus mark themselves by embedding specific proteins in their outer membrane. These proteins are recognised by markers in much the same way as the virus proteins are recognised by markers, as described above. The cell is then killed, also killing all viruses inside it.