Immunity Against Viruses
Viruses are small particles which infect living cells; this makes them obligate intracellular parasites. They have no reproductive mechanisms of their own so instead must use host cells to replicate. There are two main threats for a virus; the host’s immunity and the death of the host. Both of which will typically prevent the virus from propagating.
Many viruses are able to survive within the host for a long time without causing disease to the primary host. However when this virus is transferred to a secondary host, it is possible for a lethal disease to arise as a result. (An example is the transmission of the rabies virus to man). Vaccinating against the harmful effects of a virus therefore works best in the secondary host where the virus is not well adapted.
Structure of a Virus
Particles of a virus are known as virions. Virions encapsulate the nucleic acid core, which is surrounded by a layer of lipoprotein (or protein); this is known as the capsid. The degree of complexity of a virus can differ greatly. Viruses can contain either RNA or DNA and they can be either single stranded or double stranded, any combination of these still allows the virus (once in the host cell) to replicate.
The virion is able to bind to a wide range of molecules to mediate attachment and internalisation by a host cell (by endocytosis). Once inside the cell, the capsid of the virion breaks down releasing the viral nucleic acid into the cytoplasm of the host cell. Once inside the cytoplasm, the viral nucleic acid is able to replicate and at the same time, it is also able to inhibit the production of DNA/RNA (thus protein synthesis) of the host cell.
Possible outcomes for a cell infected by a virus:
• Lytic Infection – Host cell is destroyed, this is caused by virulent viruses
• Persistent Infection – Host cell is not lysed, but virions are released slowly, over a longer period
• Latent Infection – Occurs when there is a delay between the infection by the virus and the onset of symptoms
• Transformation – Some viruses are able to transform a normal cell into a tumour cell