Final answer:
Natural killer (NK) cells identify the 'missing self' pattern, which is associated with a deficiency of MHC I molecules on a cell's surface. These molecules are normally present on all nucleated cells and signal to the NK cells that the cell is a part of the self.
Step-by-step explanation:
Natural killer (NK) cells are a type of lymphocyte in the immune system that plays a role in the body's defense against tumors and virally infected cells. NK cells have the ability to recognize missing self. This term refers to cells that lack normal displays of MHC I molecules on their surface. NK cells monitor the presence of these molecules, which are found on all nucleated cells in the body and present normal self-antigens. When a cell is infected or becomes cancerous, it often presents an altered self or may stop expressing MHC I molecules altogether, signaling to the NK cells that the cell is abnormal.
In contrast, MHC II molecules are only expressed on certain immune cells such as macrophages, dendritic cells, and B cells, and are involved in presenting antigens to T cells to initiate an immune response. Therefore, the self pattern that is identified by NK cells is missing self, which is typically associated with a lack of MHC I molecules on the surface of a cell.