Once they are made active , B cells and T cells produce memory cells . In response to pathogens some T cells , called T helper cells , produce cytokines that direct the immune response while other T cells , called cytotoxic T cells , produce toxic granules that contain powerful enzymes which induce the death of pathogen infected cells . Following activation , B cells and T cells leave a lasting legacy of the antigens they have encountered , in the form of memory cells .