Biology/Pathogen Vs. Host
In general how does a virus resist defenses of a host?
I mean, viruses can't be killed without destroying all of the host cell, but do they have mechanisms to resist immune defenses, etc.?
Resistance to and recovery from viral infections will depend on the interactions that occur between virus and host. The defenses mounted by the host may act directly on the virus or indirectly on virus replication by altering or killing the infected cell. The non-specific host defenses function early in the encounter with virus to prevent or limit infection while the specific host defenses function after infection in recovery immunity to subsequent challenges. Although the host defense mechanisms involved in a particular viral infection will vary depending on the virus, dose and portal of entry, some general principals of virus-host interactions are summarized below.
BARRIERS TO INFECTION
The host has a number of barriers to infection that are inherent to the organism. These represent the first line of defense which function to prevent or limit infection.
The skin acts a formidable barrier to most viruses and only after this barrier is breached will viruses be able to infect the host.
Lack of Membrane Receptors
Viruses gain entry into host cells by first binding to specific receptors on cells (Table 1; adapted from: Roitt, Immunology, 5th Ed).
Cell Type Infected
CR2 (complement receptor type 2)
Many cell types
Many cell types
The host range of the virus will depend upon the presence these receptors. Thus, if a host lacks the receptor for a virus or if the host cells lacks some component necessary for the replication of a virus, the host will inherently be resistant to that virus. For example, mice lack receptors for polio viruses and thus are resistant to polio virus. Similarly, humans are inherently resistant to plant and many animal viruses.
The mucus covering an epithelium acts as a barrier to prevent infection of host cells. In some instances the mucus simply acts as a barrier but in other cases the mucus can prevent infection by competing with virus receptors on cells. For example, orthomyxo- and paramyxovirus families infect the host cells by binding to sialic acid receptors. Sialic acid-containing glycoproteins in mucus can thus compete with the cell receptors and diminish or prevent binding of virus to the cells.
The ciliated epithelium which drives the mucociliary elevator can help diminish infectivity of certain viruses. This system has been shown to be important in respiratory infections since, when the activity of this system is inhibited by drugs or infection, there is an increased infection rate with a given inoculum of virus.
The low pH of gastric secretions inactivate most viruses. However, enteroviruses are resistant to gastric secretions and thus can survive and replicate in the gut.
Humoral and cellular components
Changes that occur in the host in response to infection can also help diminish virus infectivity.
Fever can help to inhibit virus replication by potentiating other immune defenses and by decreasing virus replication. The replication of some viruses is reduced at temperatures above 37degrees C.
The pH of inflammatory infiltrates is also low and can help limit viral infections by inactivating viruses.