The differentiation of Th cells is regulated at different points by antigen
acid by other factors in a complex fashion that allows impressive flexibil
ity in the T cell response generated and enables close control at multiple
points to prevent an unwanted response. Studies over the past two years hav
e uncovered several principles of this regulation, including a new apprecia
tion of the critical role of survival factors in determining the success of
the immune response. New insights into the details of CD4(+) T cell regula
tion will provide important clues as to how immune responses are regulated,
in particular the generation of effector responses and development of long
-lived immunity.