Disclosed is a system and method for a microdevice to separate blood cells based on differences in antigen expression. Specifically, cells of the same phenotype are separated based on whether or not they are activated during infection or resting. The device of the present disclosure takes a small sample of blood and provides differential cell counts that can be used to test for infection and inflammatory response. The device can be used to identify sepsis and other infections rapidly. By measuring differences in activated white cell counts such as neutrophils, the device of the present disclosure measures physiological response to infection in hospitalized patients recovering from burns, surgeries, etc.