Hemagglutinin – Function, Structure and Significance
Hemagglutinin is a surface protein of influenza viruses that enables binding to human cells and serves as the primary target for flu vaccines.
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Hemagglutinin is a surface protein of influenza viruses that enables binding to human cells and serves as the primary target for flu vaccines.
What is Hemagglutinin?
Hemagglutinin (abbreviated HA) is a glycoprotein found on the surface of influenza viruses. It is one of the most important structural proteins of the virus and plays a critical role in the infection of human and animal cells. The name derives from the protein's ability to agglutinate red blood cells (erythrocytes) -- a process known as hemagglutination.
Structure and Composition
Hemagglutinin is a trimeric protein, meaning it consists of three identical subunits that together form a functional unit. Each subunit is composed of two polypeptide chains: HA1 and HA2, linked by a disulfide bond. The protein protrudes from the viral surface in a stalk-and-head configuration. At the globular head region lies the receptor-binding site, through which the virus attaches to host cells.
Mechanism of Action
Hemagglutinin fulfills two essential functions during the infection process:
- Binding to host cells: The protein specifically recognizes and binds to sialic acid receptors on the surface of respiratory tract cells. In humans, these receptors are predominantly found in the upper airways, which explains the typical transmission via respiratory droplets.
- Membrane fusion: After binding, the virus is taken up into the cell. In the acidic environment inside the cell, hemagglutinin undergoes a conformational change that mediates the fusion of the viral envelope with the cell membrane, allowing the viral genome to enter the cell and replicate.
Relevance to Influenza Classification
Influenza viruses are classified in part based on their hemagglutinin type. To date, 18 different hemagglutinin subtypes (H1 through H18) have been identified in influenza A viruses. Differences in HA structure determine which animal species a virus can infect and how transmissible it is in humans. Well-known subtypes in humans include H1N1 (Spanish flu, swine flu) and H3N2.
Antigenic Variability and Immune Response
The immune system responds to influenza infection by producing antibodies against hemagglutinin. These antibodies can block the virus from binding to host cells, thereby preventing reinfection. However, the influenza virus continuously alters its hemagglutinin through two key mechanisms:
- Antigenic drift: Gradual, stepwise changes caused by point mutations, which reduce the ability of existing antibodies to recognize the altered virus.
- Antigenic shift: Sudden, large-scale reassortment of gene segments between different influenza strains, potentially creating entirely new subtypes capable of triggering pandemics.
This variability is why seasonal influenza vaccines must be updated each year.
Importance for Vaccines
Hemagglutinin is the primary target antigen for seasonal influenza vaccination. Most licensed influenza vaccines contain purified HA proteins that stimulate the immune system to produce neutralizing antibodies. Researchers are actively pursuing universal flu vaccines that target conserved regions of the HA protein, potentially offering broader protection against multiple influenza strains.
Diagnostic Relevance
In laboratory diagnostics, the hemagglutination inhibition (HI) test is used to detect antibodies against influenza viruses. The test measures whether a patient's serum can inhibit the hemagglutination of red blood cells by the virus. A positive result indicates prior infection or vaccination.
References
- World Health Organization (WHO): Influenza (Seasonal) – Fact Sheet. Available at: https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)
- Skehel JJ, Wiley DC: Receptor Binding and Membrane Fusion in Virus Entry: The Influenza Hemagglutinin. Annual Review of Biochemistry, 2000; 69:531-569.
- Centers for Disease Control and Prevention (CDC): How Flu Viruses Can Change: Drift and Shift. Available at: https://www.cdc.gov/flu/about/viruses/change.htm
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