Natural killer cells are thought be an important component of innate immune response, which is the immediate response of the body to any infection or any breach of the human body surfaces. Natural killer cells produce messenger substances at the site of infection to recruit other components of the immune system to aid in the removal of the infection. So it has been assumed that having an abundance of natural killer cells will increase the likelihood of the removal of infection. But Helmholtz Centre for Infection Research (HZI) has recently published that these types of cells can infact have the opposite effect in abundance. They had observed the infection of Listeria monocytogenes, a deadly infection that can get into the blood stream and cause death in mice and immune suppressed individuals, called listeriosis. Until now it has been believed that it is due to the ineffectiveness of the killer cells in fighting the infection causes listeriosis.

The scientists at HZI has observed that overproduction of a messenger substance called Interferon-γ (IFN- γ) by the natural killer cells can block the recruitment of neutrophilic granulocytes.  These neutrophilic granulocytes are scavenger cells that help engulf bacteria. Due to this blockage, Listeria monocytogenes do not get removed from the site of infection, so they proliferate and move through the blood stream to other parts of the body spreading the infection, and eventually this results in death. Jadwiga Jablonska-Koch a member of the “Molecular Immunology” group at HZI says that having no active natural killer cells is also detrimental to health, so a balance must be achieved. It must be high enough to recruit scavenger cells to site of infection, and it should not be too high enough to cause a blockage and not let the scavenger cells an access to the bacteria. Jablonska-Koch further goes on to say that more research on the function of Natural killer cells could be essential in the treatment of bacterial infections in future.