Natural killer cells or NK cells are a type of cytotoxic lymphocyte critical to the innate immune system. The role NK cells play is analogous to that of cytotoxic T cells in the vertebrate adaptive immune response. NK cells provide rapid responses to viral-infected cells, acting at around 3 days after infection, and respond to tumor formation. They have the ability to recognize stressed cells in the absence of antibodies and MHC, allowing for a much faster immune reaction. They were named “natural killers” because of the initial notion that they do not require activation to kill cells that are missing “self” markers of MHC class 1. In addition to the knowledge that natural killer cells are effectors of innate immunity, recent research has uncovered information on both activating and inhibitory NK cell receptors which play important function roles including self tolerance and sustaining NK cell activity.

In the present book, fourteen typical literatures about NK cells published on international authoritative journals were selected to introduce the worldwide newest progress, which contains reviews or original researches on medical science, cytology, immunology, virology, etc. We hope this book can demonstrate advances in NK cells as well as give references to the researchers, students and other related people.