Life-or-death fate in the adaptive immune system.

The tissue homeostasis is essentially realized through a precise control of cellular proliferation and death. The constant balance between expansion and contraction of different cell populations is a critical hallmark of the mammalian adaptive immune system. Immune-competent cells have to confront the survival-or-demise dilemma in primis during ontogenesis and, thereafter, in their mature life in order to maintain homeostasis, since the cellular proliferation occurring during the immune response must be counterbalanced by programmed death as the immune reaction attenuates. The programmed cell death is mainly realized through apoptosis. Perturbations in precise control of lymphocyte life-or-death balance may lead to pathological processes, such as immunodeficiencies, autoimmunity or lymphoproliferative disorders. The life-or-death fate in the human adaptive immune system is reviewed and discussed.