The Host's Defenses - the Immune System
Viral infections are a considerable burden on their hosts, as they kill cells and disrupt the normal functioning of an organism. Complex organisms thus dedicate a great deal of energy to attacking viral infections in order to survive. Like most creatures, human beings have developed physical barriers - the skin, for example - which prevent viral entry. We also synthesize special enzymes which attack foreign matter (tears contain such an enzyme). Chief among these defenses, however, is the immune system. It is the immune system which allows us to live safely amidst billions of foreign and often hostile organisms.
The human immune system is built around specialized cells, called white blood cells. There are 3 types of white cells: B-cells, and two types of T-cell, CD4 and CD8, which have very different functions.
B-cells produce antibodies - special proteins which recognize receptors on the protein coat of a virus and bind to them, effectively neutralizing the virus (it can no longer attach to a cell and reproduce). B-cells bombard viral particles with antibodies until they find one which works, and then manufacture large quantities of the effective antibody and release it into the blood . They also retain a memory of these antibodies - if the virus ever attempts to reinfect the body, B-cells quickly manufacture the remembered antibody and wipe it out. This is called developing an immunity to a virus. It is for this reason that many diseases are a once-in-a-lifetime occurance, like measles or the mumps. A viral infection can only progress when the immune system is unable to
successfully arrest its progress with antibodies. This has become the logical basis for vaccines: by introducing a seriously weakened or killed version of the infectious agent, the body can safely develop antibodies to it in advance - and when the live virus comes along, kill it before it causes any serious harm.
The two types of T-cells are CD8 - often known as a "killer T-cell" - and CD4 - usually known as a "helper T-cell".
Killer T-cells recognize and kill foreign organisms and cells which have been infected by viruses. By destroying infected cells, CD8 T-cells prevent viruses from completing their reproductive cycle and producing thousands of new viruses.
Helper T-cells, create special proteins required for the production of antibodies by B cells and for the activities of the killer T-cells. They thus help in the functioning of both arms of the immune system. HIV, the virus which causes AIDS, operates by attacking and killing helper T-cells,
weakening the immune system to the point where it can no longer effectively resist infection.