Regional specialization in the mucosal immune system: primed cells do not always home along the same track.

According to the current paradigm of lymphocyte trafficking, primed B and T cells extravasate in the intestinal lamina propria chiefly by means of the mucosal homing receptor alpha4beta7, which interacts with the vascular addressin MAdCAM-1. However, as discussed here, this mechanism cannot explain the preferential homing of B cells with a high level of J-chain expression to mucosal effector sites outside the gut.