A viral antigen is a toxin or other substance emitted by a virus that elicits an immune response in its host. A viral protein is an antigen specified by the viral genome that can be detected by a specific immune response.
Morphology and viral structure
Viruses are complexes that consist of proteins and an RNA or DNA genome. They lack both cellular structure and independent metabolic processes. They replicate only by exploiting living cells based on information from the viral genome.
A mature virus particle is also known as a vision. It consists of two or three basic components (Figure 1):
1. A DNA or RNA genome, double-stranded or single-stranded, linear or circular, and in some cases segmented. A single-stranded nucleic acid can have positive or negative polarity.
2. The capsid, virus-encoded proteins that enclose the nucleic acid of the virus and determine its antigenicity; The capsid can have a cubic (rotational), helical or complex symmetry and is made up of subunits called capsomeres.
3. In some cases, an envelope surrounds the capsid and is always derived from cell membranes.
Other components of viral particles:
4. Various enzymes. Viruses require a number of different enzymes depending on the type of genome and the mode of infection. In several species of viruses, enzymes are a component of the virus particle, for example, the neuraminidase required for invasion and release of myxovirus.
Other examples include nucleic acid polymerases such as RNA-dependent RNA polymerases in antisense viruses, DNA polymerases in smallpox viruses, and RNA-dependent DNA polymerase in hepatitis B viruses and retroviruses.
5. Hemagglutinin. Some viruses (especially myxoviruses and paramyxoviruses) are capable of agglutinating several different human or animal red blood cells. These viruses carry a certain surface protein (hemagglutinin) in their envelope that allows them to do this.
The phenomenon of hemagglutination can be used for quantitative virus testing or, in the hemagglutination inhibition test, for virus identification and antibody identification. In biological terms, hemagglutinin plays a decisive role in the adsorption and penetration of the virus into the host cell.