Because they recognize a glycolipid antigen produced by a molecule called CD1d. There are lots of different types of T-cells, but the ones that do the killing of antigens that are foreign to the system are natural killer T-cells, or NKT cells. When they recognize this specific type of antigen, it is like a scent to a hunting dog, a thing used to track, lock on to and destroy. T-cells serve an invaluable service to the body by eliminating things that would ultimately cause sepsis of the whole organism, uspsetting the tender balance between the body systems involved in homeostasis of the organism. They are a part of our bodys immune system that works automatically, barring any disease, such as AIDS, that upsets the mechanism by which this natural defense system operates. AIDS, or Acquired Immune Deficiency Syndrome, is a disease that interrupts the chain of command between all the lther lymphocytes and the T-cells, which ultimately eliminate the foreign invaders that would do the body harm.