The mature T cell repertoire has the ability to orchestrate immunity to a wide range of potential pathogen challenges. This ability stems from thymic development producing individual T cell clonotypes that express T cell receptors (TCR) with unique patterns of antigen reactivity. The antigen specificity of TCRs is created from the combinatorial pairing of one of a set of germline encoded TCR variable (V) alpha and V beta gene segments with randomly created CDR3 sequences.