Professor Saltzman talks about cell communication, specifically ligand-receptor interactions that are important in maintaining homeostasis in the body. Different types of receptors and ligands, the nature of their interactions and ways to apply this into developing drugs are discussed (eg. Aldopa, Taximofen, beta-blockers). Next, Professor Saltzman talks about kinases, phosphatases, cyclic AMP and the mechanism of switching protein states. Three categories of cell communication signals are introduced: autocrine, paracrine, and endocrine. Finally, an example of cell communication using regulation/response to blood sugar level is presented.