Apoptosis triggered by FasR-Fas ligand binding plays a fundamental role in the regulation of the immune system. Its functions include:
*T-cell homeostasis: the activation of T-cells leads to their expression of the Fas ligand. T cells are initially resistant to Fas-mediated apoptosis during clonal expansion, but become progressively more sensitive the longer they are activated, ultimately resulting in activation-induced cell death (AICD). This process is needed to prevent an excessive immune response and eliminate autoreactive T-cells. Humans and mice with deleterious mutations of Fas or Fas ligand develop an accumulation of aberrant T-cells, leading to lymphadenopathy, splenomegaly, and lupus erythematosus. 
*Cytotoxic T-cell activity: Fas-induced apoptosis and the perforin pathway are the two main mechanisms by which cytotoxic T lymphocytes induce cell death in cells expressing foreign antigens.
*Immune privilege: Cells in immune privileged areas such as the cornea or testes express Fas ligand and induce the apoptosis of infiltrating lymphocytes. It is one of many mechanisms the body employs in the establishment and maintenance of immune privilege.
*Maternal tolerance: Fas ligand may be instrumental in the prevention of leukocyte trafficking between the mother and the fetus, although no pregnancy defects have yet been attributed to a faulty Fas-Fas ligand system.
*Tumor counterattack: Tumors may over-express Fas ligand and induce the apoptosis of infiltrating lymphocytes, allowing the tumor to escape the effects of an immune response. The up-regulation of Fas ligand often occurs following chemotherapy, from which the tumor cells have attained apoptosis resistance.