A) interaction between a T cell and an antigen-presenting cell to produce a specialized cell-to-cell contact area. B) activation of a B cell to become a plasma cell. C) interaction between lymphocytes and foreign antigens to produce memory cells. D) binding of a monocyte or macrophage to antigen so that it can act as an antigen-presenting cell. E) interaction of the many cytokines produced by different immunological cells.