Book excerpt: Since the introduction of microscopy, pathologists have noted tumor infiltration by inflammatory cells and presumed that this represents the host's attempt to reject its tumor. Recent advances in the molecular biology of inflammation have revealed the signals involved in attracting inflammatory cells to tumors and, for the most part, these signals are mediated by chemokines and their receptors. Chemokines are low molecular weight proteins that attract and activate specific subsets of leukocytes to the exclusion of others.