Microfluidic cartridges for agglutination reactions are provided. The cartridges include a microfluidic reaction channel with at least two intake channels, one for an antigen-containing fluid and the other for an antibody-containing fluid, conjoined to a reaction channel modified by incorporation of a downstream flow control channel. At low Reynolds Number, the two input streams layer one on top of the other in the reaction channel and form a flowing, unmixed horizontally-stratified laminar fluid diffusion (HLFD) interface for an extended duration of reaction. Surprisingly, the design, surface properties, and flow regime of microfluidic circuits of the present invention potentiate detection of antibody mediated agglutination at the stratified interface. Antigen:antibody reactions involving agglutination potentiated by these devices are useful in blood typing, in crossmatching for blood transfusion, and in immunodiagnostic agglutination assays, for example.