Over the past decade, microfluidic technologies have significantly advanced cancer liquid biopsy, particularly in the isolation and analysis of circulating tumor cells (CTCs), CTC clusters (CTCCs), and their enumeration as prognostic biomarkers. While DNA and RNA sequencing of CTCs/CTCCs have significantly advanced our understanding of their phenotypic heterogeneity, subcellular composition, genetic alterations, and gene expression profiles, accurate protein profiling and the characterization of interactions between CTCs and immune cells within the tumor and blood microenvironments remain challenging due to technical limitations. In this talk, I will present our recent advancements utilizing microfluidics, spatial profiling technologies, and advanced proximity ligation assays such as in situ PLA (isPLA) to investigate the cellular composition of CTC clusters and, more importantly, to identify PD1/PDL1 interactions and the specific cells contributing to those interactions within CTC clusters.