$\alpha$ $\beta$ TCRs Generally Have a Binding Preference for Either Peptide:MHC Class

Essay

$\alpha$ : $\beta$ TCRs generally have a binding preference for either peptide:MHC class I or peptide:MHC class II complexes. However, on occasion, one $\alpha$ : $\beta$ TCR might actually be able to recognize either class of peptide:MHC complexes. When such an $\alpha$ : $\beta$ TCR is expressed on a CD4 T cell, it will only activate its T cell after binding to peptide:MHC class II complexes. Why is this the case?