The Cell-Mediated Immune Response involves another type of Lymphocyte - Cytotoxic T Cells. These T cells share a similar external structure as Helper T cells - the T cell receptor. The function of these T cells, however, is very different. As their name implies (cyto - cell; toxic), cytotoxic T cells kill other cells. Specifically, Cytotoxic T cells are designed to kill virally-infected cells. Let's first discuss what happens when a cell is infected by a virus.

As we have studied, viruses are able to pirate cells and use the host cell's machinery to produce new viral particles. Essentially once a cell is infected by a virus it is no longer viable for that person. However, your cells don't just "give up" - they have a mechanism that is designed to help the greater body. This mechanism involves an antigen-presenting process. When a cell is infected by a virus it presents the antigen of that virus on its surface with an MHC I molecule. Note, this is a different MHC class. The purpose of this antigen presentation is to tell the body that it has been invaded and to be on the lookout for this pathogen.

Meanwhile ....

Just as with Helper T cells and B cells, there are just a few cytotoxic T cells roaming your body designed to fight each type of potential pathogen. If a CD8 cell meets up with a virally-infected cell presenting an antigen that fits its receptor the CD8 cell becomes activated (above), much as the B cell became activated when it became an APC. A subsequent interaction with a Helper T cell would initiate a cell signaling pathway that would produce a clone of these CD8 cells. This army of activated CD8 cells is then capable of seeking out virally-infected cells that are infected with the virus it was designed to fight. Once the CD8 cell locates this cell it releases a chemical called perforin which burns a hole in the infected cell and results in cell lysis. Watch an animation of a CD8 cell releasing perforin here.

Remember, that once a cell is infected by a virus it has no chance of returning to normalcy so the destruction of this cell is in the best interest of the body. In addition, bursting open the cell releases newly formed viruses that can then be captured by macrophages in the area.

You can watch an animation of the Cell-Mediated Response here.

If you were paying attention you remember that the virally-infected cell presented its antigen with a different version of MHC. Why do you think there are two classes of MHC molecules? Think back to the "condition" of each cell and its respective MHC molecules.

Macrophages - immune cells that had engulfed and digested the pathogen - presented with MHC II
B cells - immune cells that had engulfed and digested the pathogen - presented with MHC II
Virally-infected cells - cells that were no longer in charge of their destiny - presented with MHC I

So what does the MHC class indicate to the other immune system cells? Think about this important question - check in with your classmates and discuss.