Different cell types in the immune system appear to mediate their effects by markedly different means. B lymphocytes couple information for specificity with information for function in a single long-range molecule, antibody. Major histocompatibility complex (MHC)-restricted T cells, which we have analyzed in detail, appear to recognize antigen only on the surface of cells bearing the appropriate MHC gene product. This interaction provokes the T cell to release short-range, non-antigen-specific mediators (lymphokines) that preferentially act on the target cell bearing the antigen and stimulating the T cell. Regulatory T cells appear to make antigen-specific long-range molecules that, like antibody, combine specificity with information for function. However, unlike antibody molecules, these regulatory T cell products display recognition for particular target cells in the form of genetic restrictions. These behaviors are compared to strategies of cell:cell communication in the nervous system.