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Influenza Infection and Immune Response

Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. It is characterized by symptoms such as fever, cough, sore throat, muscle pain, and fatigue. Influenza viruses are divided into four types: A, B, C, and D. Influenza A and B viruses are responsible for seasonal epidemics in humans, with Influenza A also capable of causing global pandemics. Influenza A viruses are further classified into subtypes based on surface proteins hemagglutinin (H) and neuraminidase (N), such as H1N1 and H3N2. Influenza B viruses are divided into two lineages, B/Victoria and B/Yamagata. Influenza C and D viruses cause milder illness or primarily affect animals. The immune system responds to influenza by recognizing surface antigens like hemagglutinin and neuraminidase, triggering antibody production. Understanding influenza infection and immune response is crucial for developing vaccines and treatments to prevent and manage outbreaks.

Figure (1)

Structure of the influenza virion. The hemagglutinin (HA) and neuraminidase (NA) proteins are shown on the surface of the particle. The viral RNAs that make up the genome are shown as red coils inside the particle and bound to ribonucleoproteins (RNP).

Influenza viruses cause the flu by infecting the respiratory tract. These proteins help the virus attach to and exit host cells. The immune system responds by making antibodies that target these surface proteins.

When the virus changes, like through mutations or reassortment, it can avoid these antibodies. This is why flu vaccines need to be updated each year to match the most common strains. Understanding how the virus changes and how the immune system reacts helps scientists create better vaccines and treatments.

The immune system uses antibodies to fight influenza. These antibodies bind to the virus's surface proteins, especially hemagglutinin and neuraminidase. This stops the virus from infecting new cells.

However, if the virus changes its proteins through processes like antigenic drift or shift, the antibodies may no longer recognize it. Antigenic drift is when small changes build up over time. Antigenic shift is a sudden, big change that happens when different virus strains mix.

These changes can lead to new strains that spread easily and cause outbreaks. Scientists track these changes to update vaccines and protect people from the most common strains each season. This is why flu vaccines are made to include the best guesses for which strains will be most common.

Flu vaccines are made to protect against the most common strains each year. They include one A(H1N1) strain, one A(H3N2) strain, and one or two B strains. These vaccines help the body make antibodies that can fight the virus.

Influenza viruses are also studied using tools like hemagglutination inhibition assays, which test how well antibodies block the virus from clumping red blood cells.

Key Points

  • Hemagglutinin is a glycoprotein found on the surface of influenza viruses that binds to sialic acid residues on host cell membranes to initiate virus docking and infection.
  • Neuraminidase is an enzyme on the surface of influenza viruses that cleaves sialic acid residues, aiding in the release of new viral particles from infected cells.
  • Sialic acid is a type of sugar residue found on the surface of host cells that influenza viruses bind to through hemagglutinin to initiate infection.
  • An antigen is a molecular structure or protein on the surface of viruses, such as hemagglutinin and neuraminidase in influenza viruses, that is recognized by the immune system and can trigger an immune response.
  • An antibody is a protein produced by B cells that binds to influenza antigens like hemagglutinin and neuraminidase, neutralizing the virus by blocking its ability to bind to cellular receptors.
  • An influenza virus is a type of RNA virus that causes respiratory illness in humans and animals, characterized by surface proteins hemagglutinin and neuraminidase used for classification into subtypes like H1N1 and H3N2.

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