"Sensitivity" is the term used to describe the process by which you develop an allergy. Sensitivity is established when the tissues that form antibodies (lymphoid tissues) are stimulated to make specialized antibodies to otherwise harmless pollens, spores, etc. These antibodies fix to other specialized cells throughout the body that contain powerful defensive substances such as histamine. When the individual next is exposed to the pollen (as in the nose, for instance), the antibodies trigger the cells to secrete their defensive substances. This in turn causes the dilation of blood vessels, increased secretions of fluids, swelling of tissues, itching, sneezing, and other reactions that add up to hay fever.

The inflammation and other symptoms -- while real enough -- actually are not of the same destructive nature as those carried by more serious diseases. Removing the cause of the reaction results in immediate relief.