A Recent application of the equations of diffusion:
Effective intercellular communication distances are determined by the relative time constants for cyto/chemokine secretion and diffusion
Karl Francis and Bernahard Palsson
Proceedings of the National Academy of Sciences (US) Vol 94, pp 12258-12362, November 1997
The PNAS site is: http://www.pnas.org
All articles can be read and downloaded in .pdf format without payment until the end of 1997. Wouldn't it be nice if PNAS extended free access? (this is not a request for "spamming" PNAS). But, after all, they are going to keep the site up anyway for subscribers to PNAS. PNAS is a semi-governmental organization, and how better to spend the government's money?
Background (by this webmaster): Cytokines and chemokines are molecules (typically small proteins) that are secreted by one cell, and have an effect on neighboring cells. This effect is accomplished by cell surface receptors on the sensitive cells, that bind the cytokines and chemokines, and then transmit the fact of this binding to the interior of the cell. The mechanism by which binding causes the final effect is now an exciting field of research, but is not relevant to the present discussion of diffusion.
Conclusion of paper: Given the known properties of receptors and cyto/chemokines, the region in which a secreting cell influences receptive cells is about 250 microns, and the time constant for establishing and ceasing an effect is 10-30 minutes.
An example of a cytokine that has a local effect is IL-2. The function of IL-2 is to enable a T cell that has found a non-native antigen to communicate this to other nearby immune cells, e.g. T killer cells and potential antibody producing B cells.