Red blood cells(rbcs) in the presence of macromolecules(proteins, polymers etc.) aggregate to form stacked coin like linear structures called rouleaux. The shape selective aggregation here is not governed by the brownian motion of the monomer unit as is common with most self assembling systems but instead is dictated by the deformability of rbcs. This non brownian type shape selective self assembly of rbcs makes it an interesting system to understand assembly phenomena at the microscale. Our goal in this work is to study the effect of deformability of cells in self assembly. We have used a microfluidic system to mix red blood cell solution with a polymer solution. We have studied the kinetics of aggregation of cells under different flow rates using optical microscopy. We have developed a hydrodynamical model of cell collision that accounts for the shape and size of the aggregates. Using collision frequency of cells and probability of sticking we are relating our experimental findings with the theoretical model to gain insight into the role of deformability in assembly.