In a method for monitoring cell-to-cell interactions, a quartz crystal microbalance surface is exposed to a medium including a first cell. The first cell is exposed to a sample including a suspect cell. The first cell is activated prior to or simultaneously with the first cell exposure. Frequency and motional resistance changes versus time are measured after each of: surface exposure to the medium, first cell activation prior to the exposure to the sample, and first cell exposure to the sample; or after each of: surface exposure to the medium and simultaneous first cell activation and sample exposure. From the frequency and motional resistance changes versus time, any of i) a level of adhesion of the suspect cell to the activated first cell, ii) a type of the suspect cell, iii) a behavior or activity of the suspect cell is determined, or iv) activation of the first cell is determined.