tokens: Apoptosis produced in B cells through Fas ( APO-1 , CD95 ) triggering is regulated by signals derived from other surface receptors : CD40 engagement produces upregulation of Fas expression and marked susceptibility to Fas -induced cell death , whereas antigen receptor engagement , or IL-4R engagement , inhibits Fas killing and in so doing induces a state of Fas -resistance , even in otherwise sensitive , CD40 -stimulated targets .