POPULATION BIOLOGY OF LYMPHOCYTES: The flight for survival

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.