Conditions for Occurrence
Abortive interaction never concludes with the assembly of new virions. For the cycle to be interrupted, at least one of three basic scenarios must occur:
- Viral factor. A susceptible target cell is attacked by an incomplete agent—a so-called defective virus or defective virion.
- Cellular factor. A completely normal, full-fledged (standard) virus enters the host organism, but encounters a genetically resistant cell.
- Environmental factor. A standard virus successfully penetrates a highly sensitive cell, but external or internal environmental conditions prove non-permissive. In this situation, the process simply stalls at one of the stages.
Classification of Defective Agents
Not every viral particle can independently initiate an infectious process. Microbiology distinguishes specific groups of defective agents:
- Defective viruses. They act as independent species, but physically cannot replicate without outside help. They critically require a helper virus. A classic example: hepatitis D virus (delta agent) can replicate exclusively in the presence of hepatitis B virus.
- Defective virions. These are particles that, for various reasons, lack a portion of their genetic material. As a rule, they accumulate massively in cell populations at high multiplicities of infection (when multiple infections of cells occur).
- Defective interfering particles (DIPs). This is a specific group of defective virions whose main function is competition. They actively interfere with the standard virus, suppressing the production of full-fledged viral progeny.
Cellular Genetic Resistance
Cellular resistance is a powerful barrier against the generalization of infection. Defense mechanisms protecting uninfected, non-susceptible cells operate at various stages of interaction:
- Absence of receptors. Specific receptors are entirely absent from the plasma membrane of the cell. The virus physically cannot attach and penetrate inside.
- Translation block. Even if the genetic material penetrates inside, the cell blocks the process. It proves incapable of initiating viral protein synthesis on the viral mRNA template.
- Enzymatic deficiency. Specific tools are required to assemble viral macromolecules, primarily proteases and nucleases. If these are deficient in the cell, the synthesis of new particles becomes impossible.
Influence of Environmental Factors
Sometimes a cell is fully susceptible and a virus is fully competent, but external factors render reproduction impossible. Such causes for the interruption of the infectious process include:
- Changes in physiological parameters. An elevation in the patient's body temperature (fever) or a marked pH shift. For example, acidity changes are frequently observed in foci of acute inflammation.
- Drug action. Direct exposure to modern antiviral drugs also interrupts the cycle.
Important feature: An abortive infection is not always a final verdict for the virus. If conditions normalize and non-permissive factors are eliminated (e.g., body temperature drops or pH levels stabilize), the process may well be reactivated and transition to a productive type.