Maternal allo-recognition of the fetus. 
Immunological adjustments are needed to accommodate the close contact between two genetically different individuals, the mother and her baby, during mammalian pregnancy.  Contact occurs between fetal somatic or placental cells that enter the maternal systemic circulation or between uterine immune cells and the invading extravillous trophoblast.  Here we discuss two main types of maternal allo-recognition of the fetus.  One depends on avoidance of maternal T cells recognizing and responding to paternally-derived non-self human leukocyte antigens class I and class I allotypes.  The other is natural killer allo-recognition where maternally-inherited variable killer immunoglobulin-like receptors expressed by uterine natural killer cells bind to polymorphic fetal human leukocyte antigens-C molecules displayed by extravillous trophoblast.  Genetic studies indicate that natural killer cell allo-recognition regulates placentation and the allocation of resources to the fetus.