How does the influenza virus evade the immune system?

The influenza virus effectively evades the immune system primarily through the mechanisms of antigenic drift and antigenic shift. These processes involve changes in the virus's surface proteins, specifically hemagglutinin (HA) and neuraminidase (NA), which are crucial for the virus's ability to infect host cells and are also targeted by the immune system.

Antigenic drift refers to small, gradual mutations that occur as the virus replicates. These changes can lead to minor variations in the viral antigens, which may allow the virus to escape recognition by pre-existing antibodies in the host. This is significant because it can result in seasonal flu epidemics, where the immune system can no longer effectively recognize the virus despite previous exposure.

Antigenic shift, on the other hand, is a more dramatic change that can occur when two different strains of the virus infect the same host cell, exchanging genetic material. This can lead to the emergence of a new viral subtype that may be markedly different from previous strains, potentially leading to pandemics since the human immune system is unlikely to recognize the new subtype.

Through these mechanisms, the influenza virus adeptly alters its surface proteins to stay one step ahead of the immune response, making it a persistent and challenging pathogen.