BCR-mediated antigen internalization is the most efficient pathway for MHC class II presentation to CD4+ T cells, surpassing fluid-phase uptake by up to 10,000‑fold. Using hen egg lysozyme (HEL) complexed with antibodies D1.3 and HyHEL‑10, we show that antibody binding amplifies intrinsic conformational dynamics precisely at the epitope interface. Comparative B‑factor analysis of unbound and antibody‑bound HEL structures, together with NMR order parameters, reveals four findings: (1) antibody epitopes—including the subdominant T cell epitope HEL 112‑129—show elevated B‑factors (+22‑35%) upon complex formation; (2) these regions correspond to pre‑existing dynamic elements (S² = 0.65‑0.72); (3) immune complex size inversely correlates with global HEL B‑factors (global stabilization with local flexibility amplification, termed "focused flexibility"); and (4) HEL 112‑129 serves as a dual B‑cell/T‑cell epitope. Functionally, focused flexibility couples to FcRn‑dependent endosomal sorting: after pH‑dependent dissociation, FcRn binds the antibody Fc and directs recycling, while the conformationally primed antigen is actively trafficked to lysosomal compartments optimized for epitope‑conserving proteolysis. FcRn knockout studies confirm this pathway is required for efficient MHC II loading and CD4+ T cell activation. We propose that antibody binding creates an "optimally unstable" processing intermediate—sterically protected, locally destabilized, and FcRn‑routed—providing a structural and cellular basis for the extraordinary efficiency of BCR‑mediated antigen presentation.