A target cell is the cell that a hormone acts on when you are referring to the endocrine system. For example, the target cells of human growth hormone, a chemical messenger, are bone cells. This results in the growth of the person's body as additional bone cells are produced. When referring to the immune system, target cells are cells in the body that are not recognized as "self" and are then the recipient of the immune response. In this case, cells such as bacteria, viruses or mutated cells will be targeted by white blood cells and antibodies to be destroyed. The expression "target cell" makes one think of a cell with a bullseye to be targeted for some reaction to occur inside the body in an attempt to maintain homeostasis.