Figure 7-7 Left: Fetal Thymus; T cells strongly activated by DR alone, or strongly reactive to self antigen presented by HLA molecules, are selectively destroyed. T cells, with a weak or absent response to DR alone, or to DR+ self antigen, survive.
Center: Normal Adult Immune Reaction; T cell TCR and APC DR interaction is normally a weak or neutral signal. The presence of allo antigen serves to switch the signal to positive.
Right: Allo MLR; Allogeneic DR is sufficiently different from autologous DR to act as a positive signal with or without antigen present.