The central role of cell-cell crosstalk in the immune function. (A) In this model, each immune cell type specifically expresses a receptor for 1 pathogenic agent. The elimination of this agent, however, needs the coordinated effort of several immune cell types. This may only be achieved through cell-cell crosstalk, resulting in activation spreading. (B) As a prototypical example, in the case of a tumor or a microbe, initial recognition may involve NK-cells and DCs, respectively.