What is the Influenza virus?
The influenza virus, commonly known as the flu, is a ubiquitous pathogen that has been a significant health menace for centuries. It belongs primarily to the Orthomyxoviridae family, which comprises several strains that cause influenza in humans and animals. The influenza virus's history traces back to Hippocrates in 412 BC, who is credited with the first documentation of an influenza pandemic.
The discovery of the influenza virus came much later in the 20th century, in a fascinating scientific detective story. It began with the great flu pandemic of 1918, which brought about global cooperation in research amid the disaster. Richard Shope and Wilson Smith, inspired by the initial efforts of experimental virologist F.G. Novy, accomplished the significant breakthrough of isolating the virus in the 1930s.
The influenza virus is an RNA virus with a segmented genome, encapsulated in a protein coat and surrounded by a lipid envelope derived from the host cell membrane. It is classified into types A, B, and C, based on differences in the virus's nucleoprotein and matrix protein. Type A viruses are subtyped based on two surface proteins- hemagglutinin (H) and neuraminidase (N).
Symptoms of the Influenza Virus
The spectrum of influenza symptoms ranges from mild to severe, contingent upon the patient’s age, overall health, and presence of special risk factors. Common symptoms include fever, body aches, chills, fatigue, cough, sore throat, and sudden onset of symptoms.
Proteins of the Influenza Virus
The influenza virus contains several proteins, notably hemagglutinin (H) and neuraminidase (N), which are of significance as antigens. Antigens are substances that can stimulate an immune response, leading to antibody production.
Each subtype of influenza virus has a unique H and N antigen. The influenza virus's main antigens—hemagglutinin and neuraminidase—are glycoproteins embedded on the surface in the lipid envelope.
Hemagglutinin is a trimeric protein that mediates the virus's entry into host cells, while Neuraminidase is a tetrameric protein facilitating viral release from infected cells. Variations in these surface proteins define the subtypes of influenza A virus, such as H1N1 and H3N2.
Function of the Antigens
The primary role of these antigens is triggering immune responses. Hemagglutinin binds to target cells and facilitates viral entry, thereby acting as a potent antigen stimulating a host immune response.
Neuraminidase, inversely, cleaves sialic acid residues on the infected cell surface, promoting the release and spread of progeny viruses, acting as both an enzyme and an antigen.
Influenza Virus Antigen-related Signaling Pathways
The immune response elicited by the influenza virus relies heavily on complex antigen-related signaling pathways. Two significant pathways are the Toll-like receptor (TLR) pathway and the RIG-I like receptor (RLR) pathway.
On viral entry, the TLR on immune cells recognizes viral RNA, triggering a signaling cascade resulting in the production of type I interferons and pro-inflammatory cytokines. Simultaneously, the RLR pathway senses viral RNA in the cytoplasm, augmenting type I interferon production, priming the infection battleground.
