Hello, I'm an expert in the field of virology with a particular focus on infectious diseases. When we talk about the flu, which is short for influenza, we're referring to an illness caused by a group of viruses that are known for their ability to mutate and spread rapidly among populations. Understanding the components of these viruses is crucial for developing vaccines and treatments.
The "H" and "N" in flu refer to two key surface proteins of the influenza virus: Hemagglutinin (H) and Neuraminidase (N). These proteins play significant roles in the virus's ability to infect host cells and spread within a host.

Hemagglutinin (H) is a glycoprotein that enables the virus to bind to and enter host cells. It's responsible for the initial attachment of the virus to the host cell by recognizing and binding to sialic acid receptors on the surface of the host's respiratory tract cells. This binding is the first step in the infection process.

Neuraminidase (N), on the other hand, is an enzyme that helps the virus to spread. Once the virus has replicated inside the host cell, neuraminidase acts to cleave the sialic acid residues on the surface of the newly formed virus particles. This cleavage allows the new virus particles to be released from the host cell and go on to infect other cells.

The classification of influenza viruses into types A, B, and C is based on their core proteins. However, it's the H and N proteins that give rise to the various subtypes within these types. Influenza A viruses are particularly diverse and are further divided into subtypes based on the specific combinations of H and N proteins they possess. As of my last update, there are 18 known subtypes of H (H1 through H18) and 11 known subtypes of N (N1 through N11).

The diversity of these proteins is one of the reasons why the flu can be so unpredictable and challenging to combat. The ability of the virus to change its surface proteins through a process called antigenic drift allows it to evade the immune system's defenses. Additionally, a more dramatic change can occur through a process known as antigenic shift, where two different strains of the virus can exchange genetic material, potentially creating a new subtype against which the population has little to no immunity.

Understanding the H and N proteins is also important for vaccine development. Each year, scientists try to predict which strains of the flu will be most prevalent and include components that target these strains in the vaccine. This is done by monitoring the circulating strains and their H and N proteins to create a vaccine that provides the best possible match.

In conclusion, the "H" and "N" in flu are critical components of the influenza virus. They are the basis for classifying and understanding the different subtypes of the virus, which is essential for public health efforts to prevent and control the spread of influenza.

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Types of Influenza Viruses. ... Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes.Sep 27, 2017read more >>