Natural killer cells (NK cells) are the lymphocytes which belong to the same family as B and T cells, because they differentiate from a common progenitor. These cells detect and control early signs of cancer, and also recognize and kill virally infected cells. Specialized natural killer cells are found in placenta and are also known to play an important role in pregnancy. To enhance the immune system, natural killer cells secrete cytokines such as IFNγ and TNFα which act on other immune cells. Activating receptors recognize the molecules which are present on the surface of infected and cancer cells and switch on the natural killer cells. This book explores all the important aspects of natural killer cells. It brings forth the role of natural killer cells in disease response. In this book, using case studies and examples, constant effort has been made to make the understanding of the difficult concepts of this field as easy and informative as possible, for the readers.