The structural independence of V and C domains in antibodies, along with the large separation between these domains and putative signal-transducing molecules on the cell surface, rules out an allosteric mechanism for B cell activation. The protruding loop on the Cβ domain, which may also be present in the Cγ domain, could play a crucial role in contacting extracellular portions of CD3 molecules, facilitating the coupling of antigen recognition and signal transduction functions of the TCR-CD3 complex.The structural independence of V and C domains in antibodies, along with the large separation between these domains and putative signal-transducing molecules on the cell surface, rules out an allosteric mechanism for B cell activation. The protruding loop on the Cβ domain, which may also be present in the Cγ domain, could play a crucial role in contacting extracellular portions of CD3 molecules, facilitating the coupling of antigen recognition and signal transduction functions of the TCR-CD3 complex.