Answer to Question 1
T-helper cells are immune cells that act to direct and amplify the immune response through the use of chemical messengers. They have no ability to engage in phagocytic or cytotoxic actions but can direct other cells to do so. A different type of T cell, the T-cytotoxic cell, does engage directly in the fight because like the NK cell, it is licensed to kill. If you think of an NK cell as like the innate immune system's equivalent of a 007 lymphocyte, you could think of a T-cytotoxic cell as the adaptive immune system's 008 lymphocyte.
Answer to Question 2
Another very important defender against the bad characters are the natural killer (NK) cells that are lymphocytic cells programmed to recognize other cells that are non-self such as tumor cells and cells infected with viruses, and to release cytotoxic (poisonous to the cell) chemicals that lyse (i.e., chemically degrade and destroy) these cells. These cells are important first defenders against endogenous threats such as tumors and cancers. They are like James Bond special agents that are licensed to carry specially designed poison dart weapons that neutralize domestic villains.
Occurrence of stem cells at different stages of mammalian development
Prokaryotic and Eukaryotic Cells
Allogeneic stem cells
Characterization of canine stem cells (MSCs).
Activation and Clonal Selection of a Cytotoxic T Cell
Lederberg and Tatum’s detection of recombination between auxotrophic E. coli cells