It is also clear that homeostasis of lymphocytes involves active participation by resting lymphocytes themselves. We have recently shown that resting T cells divide vigorously after transfer to hosts that lack T cells. This division is not caused by engagement of the antigen receptors on the resting cells. Rather, it seems to be a response to factors that accumulate in animals that lack T cells. Future experiments will try to find out what these factors may be.
T cells are even more stringently controlled after they have encountered antigen.
Again, the findings of the past 10 years have contradicted long-held beliefs. Thus it was once thought that encounter with antigen would cause antigen-specific T cells to divide rapidly. Although a few of these cells might then die, it was thought that after antigen had disappeared many of the antigen-specific T cells would remain in the host and form the pool of memory T cells that would help to protect the animal against a second attack by organisms bearing that same antigen. Experiments by our laboratory and others have disproved this. Exposure to antigen does indeed make the antigen-specific T cells divide.