The diffusive transfer, or paracrine delivery, of chemical factors during the interaction of an emitting cell and a receiving cell is a ubiquitous cellular process that facilitates information exchange between the cells and/or to bystander cells. In the cellular immune response this exchange governs the magnitude and breadth of killing of cellular targets, inflammation or tolerance. Paracrine delivery is examined here by solving the steady-state diffusion equation for the concentration field surrounding two intensely interacting, equi-sized cells on which surface kinetics limits the rates of factor emission and absorption. These chemical factors may be cytokines, such as Interlukins and Interferons, but the results are presented in a generic form so as to be applicable to any chemical factor and/or cell-type interaction. In addition to providing overall transfer rates and transfer efficiencies, the results also indicate that when the receiving cell is naive, with few factor receptors on its surface, there may be a significant accumulation of factor in the synaptic region between the cells with a consequent release of factor to the medium where it can signal bystander cells. This factor accumulation may play a critical role in activating a naive receiving cell. As the receiving cell activates and becomes more absorbent, the factor accumulation diminishes, as does potential bystander signaling. (C) 2012 Elsevier Ltd. All rights reserved.