The adaptive branch of the immune system is built on the principle that the enemy can take any shape or form, but will display different protein sequences from the host. A highly flexible mechanism has evolved to recognize foreign protein segments based on a developmentally programmed series of transient cell–cell synapses between T cells armed with T cell antigen receptors (TCR) and antigen-presenting cells (APC) decorated with peptide ligands. The ligands are composed of peptides held in a surface groove on major histocompatibility complex molecules (MHC–peptide) that together form an interacting surface for the TCR (