One striking feature of TCR interactions with peptide:MHC complexes is that amino acid residues in the MHC protein are as important to the TCR binding strength as are amino acid residues in the pathogen-derived peptide. This feature is in contrast to antigen recognition by antibodies, which is a direct interaction that is independent of other host proteins. Based on the different functions of T cells versus antibodies in the adaptive immune response, the fact that TCRs recognize components of both the MHC and the bound peptide exists to: