Biological systems function through coordinated interactions between cells. From early development to adult tissue maintenance, most physiological processes depend on cells communicating with and responding to one another. Individual cells carry out specialized functions, but it is their coordination that gives rise to coherent biological behavior. This is particularly evident during organismal development, where cell fate decisions depend on signals exchanged between neighboring cells and their environment. Similar principles apply in adult tissues, where continuous communication maintains structure, function, and balance. When these interactions are disrupted, normal tissue homeostasis is often lost.

Immune function represents a particularly interaction-driven system. Immune cells constantly sense, interpret, and respond to signals from other cells. Activation, suppression, differentiation, and memory formation all depend on coordinated communication rather than on isolated cellular states. Cells communicate through multiple mechanisms. Some interactions are mediated by soluble factors such as cytokines, chemokines, or growth factors that act over short or long distances. Others require direct physical contact, involving receptor-ligand engagement at the cell surface or the formation of structured contact sites. In many cases, these modes of communication operate simultaneously and influence one another.

Therefore, what matters biologically is not only which signals are present, but when and where they occur, how long they persist, and in what cellular context they are interpreted. Two cells with similar molecular profiles can behave very differently depending on how they interact with surrounding cells. Despite the central role of cell-cell interactions in shaping biological function, much of experimental biology has focused on characterizing cells in isolation. This focus has been driven in part by technical feasibility. Individual cells are easier to profile, classify, and compare than dynamic interactions that unfold over time. The consequence is a gap between how biology operates and how it is often measured. Cellular nteraction-driven processes are widely acknowledged in principle, but they remain difficult to capture systematically and at scale. This tension becomes particularly visible in applied fields that rely on accurate functional readouts, including cell- and immunotherapies.