The Centers for Disease Control and Prevention (CDC) recommends getting a flu vaccine every year, since the influenza virus is constantly mutating. These vaccines help produce antibodies that will fight off particular strains of the virus, preventing them from dominating the immune system. In a new study, researchers have found out exactly how the virus destroys the immune system’s first line of defense, known as B cells, and spreads throughout the body.

When the flu enters the body, the immune system produces virus-specific B cells, which are capable of creating antibodies. These cells’ receptors are able to bind onto virus particles, and reduce viral spread. They also carry information about the virus and its antibody to the lungs, where they’re able to fight off any other exposure to the virus through inhalation. But as influenza enters the body, it attaches to the B cell receptors and uses the receptors’ specificity to gain entry and block antibody production. This process allows the virus to kill off the first wave of B cells and spread before the immune system can produce a second wave.

“This is how the virus gains a foothold,” Stephanie Dougan, co-author of the study, and postdoctoral researcher at the Whitehead Institute of Biomedical Research, said in a statement. “The virus targets memory cells in the lung, which allows infection to be established — even if the immune system has seen this flu before.”

This is just one of the ways influenza establishes infection, Joseph Ashour, another co-author of the study said. Children are most at risk for getting the flu due to their still-developing immune systems. Every year, about 20,000 children under five years old are hospitalized for complications related to the flu, according to the U.S. Department of Health & Human Services. Meanwhile, some strains of the virus are particularly deadly, such as H1N1, also known as the swine flu, which killed between 9,000 and 18,000 deaths from 2009 to 2010.

The researchers discovered the infection process by using a fluorescent protein-labeling technology to identify the influenza virus. Seeing where the virus travelled allowed them to isolate the B cells as they grabbed ahold. Then, using the B cells’ nuclei, and a cloning technique, they were able to produce mice that had influenza-specific B cells at levels high enough to track the virus’s interactions.

While the results offer new insights into the spread of the flu virus within the body, the researchers say that their model can be applied to other viruses as well. “We can now make highly effective immunological models for a variety of pathogens,” Dougan said in the statement. “This is actually a perfect model for studying memory in immune cells.”

Source: Dougan S, Ashour J, Ploegh H, et al. Antigen-specific B-cell receptor sensitizes B cells to infection by influenza virus. Nature. 2013.