Fighting Viral Infections
The fact that viruses reproduce within host cells means that the immune response to viral infection is very different to the way in which the body responds to bacterial infection. The mechanisms that defend against bacterial infection act by attacking the bacteria directly, whereas viral immunity depends on the ability of the immune system to kill host cells. Obviously, killing host cells would be lethal to the organism as a whole if it were not highly controlled. The main cell for fighting viral infection is the cytotoxic T-lymphocyte (or Tc cell). It is referred to by several names such as killer T cell, killer T lymphocyte, CD8 cell etc. All these names are synonymous.
T-lymphocytes
B-lymphocytes have an antigen receptor which is an IgM molecule. This receptor is able to bind to free antigen; that is antigen free in plasma (such as a toxin) or antigen on the surface of a pathogen. Similarly T-Lymphocytes have an antigen receptor on their surface. It is rather unimaginatively named the T-cell receptor (sometimes abbreviated to TcR). The T-cell receptor is part of the immunoglobulin super-family and thus is a glycoprotein. Unlike the B-lymphocyte receptor it is unable to bind to free antigen. The T-cell receptor binds to antigen that is presented to it. This presentation is by a molecule that is referred to as the MHC. T-lymphocytes are said to be MHC-restricted.
MHC
The Major Histocompatibility Complex is central to the functioning of T-lymphocytes and therefore to the ability of the immune system to fight viral infection. There are two classes of MHC: MHC-I and MHC-II. MHC-I is found on all nucleated cells and binds to the T-cell receptor on Tc cells. The MHC-II molecule is only found on specific cells known as antigen-presenting cells. It binds to the T-cell receptor on T-helper cells. T-helper cells are important in both bacterial and viral immunology and their function is described in the section on T-helper cells and HIV. The MHC is discussed in the next section.
Cytotoxic T-lymphocytes
Cytotoxic T-lymphocytes have a very simple function: they bind to viral antigens presented by infected cells on their MHC-I molecules. As a consequence of this binding, the T-cell kills the infected cell by a number of mechanisms. As such Tc cells are the main means of killing viruses.
Natural Killer Cells
NK cells are not well understood but they do play a role in fighting viral infection. It is thought that NK cells are most active early in infection before the adaptive response, which takes time to become active. NK cells are part of the innate immune system and are able to kill virally infected cells. Their role and function is discussed in this section.
The Major Histocompatibility
Complex