Natural Killer (NK) cells are immune cells with an important role in the first line of defense against cancer formation, metastases (spread) of tumour cells, and infections by viruses and other pathogens. Once known solely for their role in the innate immune system, responding to pathogens in an immediate and non-specific way, research now suggests that certain NK cells might also be involved in regulating the more targeted adaptive immune response. It is believed that a subset of NK cells in the lymph node is largely responsible for this function. With new knowledge about subsets of NK cells that have specialized functions, researchers are now looking at how these NK cells arise. It is possible that in addition to the NK cells that develop in bone marrow, other immature NK cells travel to different parts of the body where they mature in specific microenvironments that affect their function. In the lung this could mean a better NK cell response to cancer metastases or virus infection, whereas lymph node NK cells might be better at interacting with other immune system components. Timotheus Halim’s research seeks to find immature NK cells (NK cell progenitors) in sites other than the bone marrow. NK cell progenitors have already been found in the lymph node and lung. Now, Halim is using different mouse models to evaluate how important these progenitors are in forming mature NK cells. He is also determining if the NK cells that arise from these novel NK cell progenitors have specialized functions. A better understanding of NK cell development and function is critical in understanding the overall management of the immune system. Ultimately, this knowledge could help in the development of immunotherapy and other forms of treatment against cancer and infection.