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Background Cell therapies are a promising approach to treat many types of cancers but balancing the tumor-specific potency while mitigating toxicities remains a major hurdle to efficacious, safe treatments. Conventional in vitro methods to characterize how cell therapies bind and kill tumors include characterizing molecular affinity, and downstream functional assays such as cytokine secretion and cytotoxicity. There is still a lack of mechanistic understanding how to tune cell-cell interactions that drive functional processes important to selecting the best therapy.Methods Cell avidity, the collective strength of interactions between an effector and its target, can help elucidate the mechanism of action for therapeutic candidates. Our highly flexible Cell Avidity platform, the z-Movi Cell Avidity Analyzer, combines microfluidic-based fluorescence microscopy with acoustic force to quantify cell-cell and cell-protein binding in a physiological context. This provides a comprehensive view of cell binding characteristics to interrogate binding potency, safety, sensitivity, and kinetics.Results Here, we review recent publications highlighting how researchers have used Cell Avidity to:Format-tune in vivo-launched bispecific T cell engagers to enhance efficacy against renal cell carcinoma.Treat NK cells with Venetoclax to improve cytoskeleton remodeling and lytic granule polarization, contributed to a more efficient IS, enhancing NK cell-mediated killing of AML cells.Engineer CAR-T cells secreting a T-cell engaging molecule to overcome a challenging tumor microenvironment in pancreatic adenocarcinoma.Fine-tune the affinity/cell avidity of CAR-T cells to mitigate on-target off-tumor toxicity in renal cell carcinoma.Conclusions Cell Avidity is an emerging biomarker that directly quantifies the strength of the physical interactions between target and effector cells. We developed a Cell Avidity platform that enables the characterization and screening of molecular binders and cellular products, including antibodies, small molecules, and cell therapies. Cell Avidity provides comprehensive information on potency, safety, sensitivity, and kinetics, offering key insights into the mechanism of action for cell therapies.