How Viruses Cause Disease
A single virus particle (virion) that infects a single host cell can produce thousands of progeny in the infected cell. The cell often breaks open (lyses) and thereby allows the progeny viruses access to nearby cells. Many of the clinical manifestations of viral infection reflect this cytolytic effect. For example, the cold sores formed by herpes simplex virus and the lesions caused by the smallpox virus reflect the killing of the epidermal cells in the local area of infected skin.
About 15% of all malignant cancers are also cause by viruses. Hepatitus B virus, for example, is associated with liver carcinoma. Certain types of papillomarviruses are associate d with cervical and penile cancer. These types of papillomaviruses exist as an episomal viral DNA and integrate into the host DNA thereby disrupting the negative regulator E2 resulting in strong activation of E6 and E7 oncogenes that bind and inactivate p53 and Rb.
Just as the genotype of the invading virus can influence the course of a viral infection, so too can the genetic background of the invaded host. For example, mousepox is a generalized infection caused by the ectroemlia virus, whihc is an inapparent disease in the genetically reistanct C57B1/6 mouse strain. On the other hand, only one infectious particle of the same virus can result in 100% mortality in the sensitive BALB/c or A mouse strains. This difference in strain susceptibility is related to differences in the cytokine profile produced after infection. In the C57B1/6 strain, there is a rapid induction of type 1 cytokiens and a potent cytotoxic T lymphocyte resposne, whereas in the BALB/c or A mice these cytokines are virtually absent with a complete absence or delayed induction of a CTL response.
How are Viruses Detected by the Immune System?
Virus Identification and Detection
Some ways to identify and detect viruses include the following:
- PCR where nucleic acid is extracted from a specimen (if RNA virus, RNA is converted to cDNA by reverse transcriptase) viral DNA is amplified by virus-specific primer pairs and DNA fragment is analyzed by agarose gel electrophoresis.
- ELISA where viral antigen is bound to plastic surface and antibodies in the patient's sera specifically bind the viral antigen. This antigen-antibody complex is then reacted with anti-human IgG conjugated with an enzyme (e.g., alkaline phosphatase). This complex is then visualized by adding chemical colorigenic substrates.
- Western Blot is a more confirmatory assay than ELISA for HIV. Viral proteins (e.g., gp120, gp41 and p24 in the case of HIV) are transfered to filter and filter is incubated with patient's serum. Filter is then incubated with labeled anti-human Ig serum.
