Dr. Greene's Answer
Two types of defense against viruses predominate in the bloodstream: humoral immunity and cellular immunity. The humoral (or one might say ‘liquid’) immune system attacks viruses when they are loose in the body, either in the bloodstream or in bodily secretions. The cellular immune system attempts to destroy viruses once they have taken up residence inside the body’s cells.
The humoral response consists of antibodies made to specific viruses. These antibodies remain present in the circulation and secretions, hopefully eliminating the virus and protecting against future infections. The more water soluble a particular virus is, the more effective the humoral response. A good example of this is the poliovirus. Polio vaccines (and other vaccines) work precisely because they so effectively stimulate specific antibody formation. When a person is re-exposed to polio, antibodies destroy the virus before infection sets in.
The cellular response consists of certain white blood cells, such as cytotoxic lymphocytes or natural killer cells, which attack and destroy our own cells that have been altered by viruses. Some viruses, such as herpes, are ‘sneaky’ enough to hide in our cells without changing the way they look to the cellular immune system. These viruses can remain dormant within cells for years, only to re-emerge periodically when our humoral defenses are weak and allow the viruses to get loose in the circulation once again.