The 2-Signal Model of Lymphocyte Activation Absence of Signal 2 T Cell APC TCR MHC Tolerance Clonal Anergy or Deletion Signal 1+Signal 2 CD28 B7 APC T Cell TCR MHC cytokines Activation BDC Absence of Signal 2 Activation Clonal Anergy or Deletion TCR MHC APC Tolerance T Cell TCR MHC APC Signal 1 + Signal 2 B7 T Cell cytokines CD28 The 2-Signal Model of Lymphocyte Activation BDC