In the past few years a considerable number of studies have focused on the mechanisms of antigen presentation by classical major histocompatibility complex (MHC) class I and class II encoded molecules. Among different approaches, the engineering of recombinant chimeric genes and proteins has provided new tools to analyze the parameters influencing the intracellular processing of antigenic determinants. This review will summarize and discuss the different models of recombinant genes and molecules that have been used to analyze the influence of the molecular environment of a T cell determinant on its efficient processing and MHC presentation. This approach may also represent an interesting tool for developing new vaccine strategies for inducing T cell responses against pathogens.