Microfluidic systems provide a unique set of tools for manipulating the environment of live cells while under continuous observation by optical microscopy. This talk will describe the use of a simple microfluidic system to follow the activation of B cells upon exposure to subsaturating doses of ligands by fluidic switching. The timing and sequence of these pulses makes it possible to observe temporally-distinct populations of receptors. It is possible to track the rearrangement and internalization of surface-expressed B cell receptors (BCRs) on primary naïve B cells. Ratiometric imaging also allows kinetic measurements of the activation following stimulation.