In this essay we suggest that the primary goal of the cells of the immune s
ystem is to ensure their own growth and survival. In adults, in steady-stat
e conditions, the number and distribution of lymphocyte populations is unde
r homeostatic control. New lymphocytes that are continuously produced in pr
imary and secondary lymphoid organs must compete with resident cells for su
rvival. We discuss recent findings supporting lymphocyte survival as a cont
inuous active process and implicating cognate receptor engagement as fundam
ental survival signals for both T and B lymphocytes. The conflict of surviv
al interests between different cell types gives rise to a pattern of intera
ctions that mimics the behavior of complex ecological systems. In their fli
ght for survival and in response to competition, lymphocytes use different
survival signals within different ecological niches during cell differentia
tion. This is the case for T and B lymphocytes and also for naive and memor
y/activated T and B cells. We discuss how niche differentiation allows the
co-existence of different cell types and guarantees both repertoire diversi
ty and efficient immune responses.