A channel-cell system is provided for detecting the presence and/or measuring the presence of analyte particles in a sample stream comprising: a) a laminar flow channel; b) two inlets in fluid connection with the laminar flow channel for respectively conducting into the laminar flow channel (1) an indicator stream which may comprise an indicator substance which indicates the presence of the analyte particles by a detectable change in property when contacted with the analyte particles, and (2) the sample stream; c) wherein the laminar flow channel has a depth sufficiently small to allow laminar flow of the streams and a length sufficient to allow particles of the analyte to diffuse into the indicator stream to the substantial exclusion of the larger particles in the sample stream to form a detection area; and d) an outlet for conducting the streams out of the laminar flow channel to form a single mixed stream.