The innate immune system recognizes infectious agents such as viruses and bacteria. A group of lymphocytes known as "innate lymphoid cells" or ILCs plays a central role in the defense of the human body against infective agents, researchers have found. They have discovered previously unidentified ILCs that are able to protect epithelial surfaces, such as those of the intestinal mucosa, against infection. The results provide important additional insights into how the immune system functions.