Abstract
Adaptive immunity requires both specific recognition of an antigen and its translation into appropriate lymphocyte responses. This paper reviews the striking differences in B- and T-lymphocyte antigen recognition, details the pathways for conversion of protein antigens into peptide–major histocompatibility complex molecule ligands for T-cell receptors, and summarizes the roles of costimulatory signals in lymphocyte activation. This information is used to suggest new approaches for the rational design of vaccines.