We discussed the need for cellular communication, accomplished through endogenous messengers which are chemical, produced by cells, and delivered into to the extracellular fluid and the importance and definition of the target cell producing a specific effect. Histamine was used as an example of a chemical messenger produced in response to a pathogen. Steps from the need for white blood cells to attack the pathogen, to histamine causing an increase in capillary permeability allowing a greater concentration of wbc's to pass through the capillary walls and attack the pathogen was outlined. Other effects of histamine were listed and anaphylactic shock discussed, along with emergency treatment. The chemistry of histamine was looked at as a preview to understanding the use of antihistamine.