The Role of NK Cells in Implantation after IVF and Treatment Strategies

16.1 BackgroundThere are three types of lymphocyte, T cells, B cells and natural killer (NK) cells. T and B cells belong to the highly specific adaptive immune system, whereas NK cells are the lymphocytes of the innate immune system. In humans, a combination of the surface markers CD56 and CD16 identifies all the various subtypes of NK cells. NK cells in the blood (pbNK) recognize virally infected and cancerous cells, providing innate immunity through continuous peripheral surveillance. The name, natural killer, is derived from their ability to respond rapidly and lyse target cells in the absence of antibodies and human leukocyte antigen (HLA) molecules utilized by other lymphocytes. NK cells recognize either altered (eg virally stressed) or absent/foreign (eg allogeneic) expression of HLA on target cells through the identification of major histocompatibility complex (MHC) class I molecules (HLA in humans).Cells belonging to the NK lineage exist in many organs, providing a wide and varying range of functional roles. Human NK cell populations are currently well described in the blood (majority are CD56dimCD16+) and in a range of tissues, including lymph nodes, thymus, spleen, liver and uterus (uNK, CD56superbrightCD16−). It is important to note that whilst these cells probably originate from common progenitors, broad differences exist in the phenotypical markers and physiological roles between most tissue NK subtypes. There are many mechanisms in humans and murine models that describe how mothers tolerate a genetically unfamiliar, invading conceptus. Since this notion of maternal tolerance to the semi-allogenic conceptus was first …