"The discovery comes from detailed studies of the transmembrane protein straddling the worlds inside and outside of lymphocytes and other cells of the immune system. These receptor proteins are like spokes piercing the cell membrane. Part of the receptor pokes into the cell and part extends out from the cell surface. The external "domain" receives signals from outside the cell which regulate responses inside -- ranging from increasing the cell's glucose supply to starting a new cell division cycle to generate more lymphocytes."