Some Viral Adhesins and Their Host Attachment Sites | |||
---|---|---|---|
Pathogen | Disease | Adhesin | Attachment Site |
Influenzavirus | Influenza | Hemagglutinin | Sialic acid of respiratory and intestinal cells |
Herpes simplex virus I or II | Oral herpes, genital herpes | Glycoproteins gB, gC, gD | Heparan sulfate on mucosal surfaces of the mouth and genitals |
Human immunodeficiency virus | HIV/AIDS | Glycoprotein gp120 | CD4 and CCR5 or CXCR4 of immune system cells |
Antigenic variation in viruses
Antigenic variation also occurs in certain types of enveloped viruses, including influenza viruses, which exhibit two forms of antigenic variation: antigenic drift and antigenic shift ( [link] ). Antigenic drift is the result of point mutations causing slight changes in the spike proteins hemagglutinin (H) and neuraminidase (N). On the other hand, antigenic shift is a major change in spike proteins due to gene reassortment. This reassortment for antigenic shift occurs typically when two different influenza viruses infect the same host.
The rate of antigenic variation in influenza viruses is very high, making it difficult for the immune system to recognize the many different strains of Influenzavirus. Although the body may develop immunity to one strain through natural exposure or vaccination, antigenic variation results in the continual emergence of new strains that the immune system will not recognize. This is the main reason that vaccines against Influenzavirus must be given annually. Each year’s influenza vaccine provides protection against the most prevalent strains for that year, but new or different strains may be more prevalent the following year.
For another explanation of how antigenic shift and drift occur, watch this video.
- Describe the role of adhesins in viral tropism.
- Explain the difference between antigenic drift and antigenic shift.
Key concepts and summary
- Virulence factors contribute to a pathogen’s ability to cause disease.
- Exoenzymes and toxins allow pathogens to invade host tissue and cause tissue damage. Exoenzymes are classified according to the macromolecule they target and exotoxins are classified based on their mechanism of action.
- Bacterial toxins include endotoxin and exotoxins . Endotoxin is the lipid A component of the LPS of the gram-negative cell envelope. Exotoxins are proteins secreted mainly by gram-positive bacteria, but also are secreted by gram-negative bacteria.
- Bacterial pathogens may evade the host immune response by producing capsules to avoid phagocytosis, surviving the intracellular environment of phagocytes, degrading antibodies, or through antigenic variation .
- Viral pathogens use adhesins for initiating infections and antigenic variation to avoid immune defenses.
- Influenza viruses use both antigenic drift and antigenic shift to avoid being recognized by the immune system.
Fill in the blank
The glycoprotein adhesion gp120 on HIV must interact with __________ on some immune cells as the first step in the process of infecting the cell.
CD4
Adhesins are usually located on __________ of the pathogen and are composed mainly of __________ and __________.
surface; proteins; sugars
The Shiga and diphtheria toxins target __________ in host cells.
protein synthesis
Antigenic __________ is the result of reassortment of genes responsible for the production of influenza virus spike proteins between different virus particles while in the same host, whereas antigenic __________ is the result of point mutations in the spike proteins.
shift; drift