Adaptive cellular immunity triggered by "foreign" antigens (virus, cancer, etc.)

PDL1 Expression by Trophoblastic cells protects from maternal rejection

Human trophoblastic cells display many of the features of malignant cells such as cellular growth and their ability to invade normal tissue including blood vessels and are apparently not eradicated by the host immune system. These trophoblastic cells are genetically significantly different from the mother and should initiate a maternal immune rejection. This is however suppressed thank to the PD1-PDL1 interaction